Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 48,138	$ 49,364	$ 32,392
Charged to Costs and Expenses	76,115	88,110	108,980
Deductions	(92,864)	(89,336)	(92,008)
Balance at End of Period	31,389	48,138	49,364
Allowance for doubtful accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	32,856	36,368	13,609
Charged to Costs and Expenses	1,235	6,079	26,225
Deductions	(15,766)	(9,591)	(3,466)
Balance at End of Period	18,325	32,856	36,368
Reserve for sales returns and allowances [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	6,121	4,639	7,214
Charged to Costs and Expenses	65,562	72,995	71,649
Deductions	(66,700)	(71,513)	(74,224)
Balance at End of Period	4,983	6,121	4,639
Reserve for unapplied rebates [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	9,161	8,357	11,569
Charged to Costs and Expenses	9,318	9,036	11,106
Deductions	(10,398)	(8,232)	(14,318)
Balance at End of Period	$ 8,081	$ 9,161	$ 8,357