DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - consolidated balance sheets for accruals and allowances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	$ 47,705
Ending balance	59,946	$ 47,705
Chargebacks
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	39,007	28,230	$ 26,785
Accruals/Adjustments	260,771	229,813	179,297
Credits Taken Against Reserve	(249,896)	(219,036)	(177,852)
Ending balance	49,882	39,007	28,230
Government Rebates
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	8,974	7,930	5,891
Accruals/Adjustments	17,549	11,383	12,237
Credits Taken Against Reserve	(17,622)	(10,339)	(10,198)
Ending balance	8,901	8,974	7,930
Returns
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	12,552	8,274	5,756
Accruals/Adjustments	19,105	14,243	12,184
Credits Taken Against Reserve	(15,062)	(9,965)	(9,666)
Ending balance	16,595	12,552	8,274
Administrative Fees and Other Rebates
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	7,353	5,226	3,550
Accruals/Adjustments	36,874	33,167	24,037
Credits Taken Against Reserve	(35,946)	(31,040)	(22,361)
Ending balance	8,281	7,353	5,226
Prompt Payment Discounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning balance	2,009	1,834	1,554
Accruals/Adjustments	10,789	9,371	8,126
Credits Taken Against Reserve	(10,249)	(9,196)	(7,846)
Ending balance	$ 2,549	$ 2,009	$ 1,834