Annual report pursuant to Section 13 and 15(d)

ACCOUNTING PRONOUNCEMENTS - Narrative (Details)

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ACCOUNTING PRONOUNCEMENTS - Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Oct. 01, 2019
Oct. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Gross profit $ 142,596,000 $ 137,178,000 $ 136,281,000 $ 126,411,000 $ 149,513,000 $ 152,615,000 $ 150,680,000 $ 131,413,000 $ 542,466,000 $ 584,221,000 $ 567,788,000    
Accounting Standards Update 2017-07                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Gross profit         $ 729 $ 714 $ 714 $ 714          
Accounting Standards Update 2016-16                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Cumulative tax adjustment for intra-entity transfers                         $ (4,176,000)
Retained Earnings | Accounting Standards Update 2016-16                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Cumulative tax adjustment for intra-entity transfers                         (4,176,000)
Retained Earnings | Accounting Standards Update 2018-02                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Cumulative tax adjustment for intra-entity transfers 8,814,000               8,814,000        
Accumulated Other Comprehensive (Loss) Income (net of tax) | Accounting Standards Update 2018-02                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Cumulative tax adjustment for intra-entity transfers $ (8,814,000)               $ (8,814,000)        
Cost of Sales | Accounting Standards Update 2017-07                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Reclassification of net benefit costs                   (2,871,000) (4,401,000)    
Selling and Marketing Expense | Accounting Standards Update 2017-07                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Reclassification of net benefit costs                   (907,000) (1,391,000)    
General and Administrative Expense | Accounting Standards Update 2017-07                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Reclassification of net benefit costs                   $ (1,945,000) $ (2,981,000)    
Other Assets | Accounting Standards Update 2016-16                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Cumulative tax adjustment for intra-entity transfers                         $ (4,176,000)
Forecast | Subsequent Event | Accounting Standards Update 2016-02                          
New Accounting Pronouncements or Change in Accounting Principle [Line Items]                          
Operating lease, right-of-use asset                       $ 85,000,000  
Operating lease, liability                       $ 85,000,000