Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Provisional tax benefit recognized for remeasurement of deferred tax liability	$ 12,000
United States		$ 259,436	$ 234,646	$ 203,269
Foreign		9,746	6,732	4,881
Total		269,182	241,378	208,150
Current [Abstract]
Federal		71,329	77,000	65,676
State and other		11,289	12,182	10,263
Current tax		82,618	89,182	75,939
Deferred [Abstract]
Federal		(6,643)	4,079	4,568
State and other		2,007	(330)	(370)
Deferred tax		(4,636)	3,749	4,198
Total		$ 77,982	$ 92,931	$ 80,137
Reconciliation of U.S. federal statutory tax rate to effective tax rate [Abstract]
Federal statutory rate (in hundredths)		35.00%	35.00%	35.00%
Change in valuation allowance (in hundredths)		(0.06%)	0.10%	0.20%
Stock-based compensation (as a percent)		(4.67%)	(0.00%)	(0.00%)
Federal tax reform (as a percent)		(4.46%)	0.00%	0.00%
Other, primarily state income tax rate (in hundredths)		3.16%	3.40%	3.30%
Total effective tax rate (in hundredths)		28.97%	38.50%	38.50%
Deferred tax assets [Abstract]
Product inventories			$ 7,010
Accrued expenses			3,978
Allowance for doubtful accounts			396
Total current			11,384
Component reclassified for net presentation			(5,368)
Total current, net			6,016
Product inventories	4,287	$ 4,287
Accrued expenses	1,804	1,804
Allowance for doubtful accounts	154	154
Leases	1,188	1,188	1,920
Share-based compensation	8,884	8,884	13,778
Uncertain tax positions	2,087	2,087	2,746
Net operating losses	5,441	5,441	5,735
Interest rate swaps	0	0	674
Other	1,475	1,475	2,465
Total noncurrent	25,320	25,320	27,318
Less: Valuation allowance, non-current	(5,440)	(5,440)	(5,735)
Component reclassified for net presentation	(19,071)	(19,071)	(20,781)
Total non-current, net	809	809	802
Total deferred tax assets	809	809	6,818
Deferred tax liabilities [Abstract]
Trade discounts on purchases			2,698
Prepaid expenses			2,670
Total current			5,368
Component reclassified for net presentation			(5,368)
Total current, net			0
Trade discounts on purchases	1,520	1,520
Prepaid expenses	1,857	1,857
Intangible assets, primarily goodwill	29,348	29,348	42,930
Depreciation	10,870	10,870	12,326
Interest rate swaps	61	61	0
Total non-current	43,656	43,656	55,256
Component reclassified for net presentation	(19,071)	(19,071)	(20,781)
Deferred income taxes	24,585	24,585
Total non-current, net			34,475
Total deferred tax liabilities	24,585	24,585	34,475
Net deferred tax liability	23,776	23,776	27,657
Loss Carryforwards [Line Items]
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities		0	7,370	$ 7,706
Uncertain tax positions activity [Roll Forward]
Beginning balance		7,846	5,978	4,690
Increases for tax positions taken during a prior period		129	10	410
Increases for tax positions taken during the current period		3,260	2,819	1,782
Decreases resulting from the expiration of the statute of limitations		869	961	904
Decreases relating to settlements		429	0	0
Ending balance	9,937	9,937	7,846	5,978
Unrecognized tax benefits that, if recognized, would decrease the effective tax rate	7,850	7,850	5,100
Interest expense related to unrecognized tax benefits		249	156	$ 136
Accrued interest on unrecognized tax benefits	866	866	685
International Subsidiaries [Member]
Loss Carryforwards [Line Items]
Tax loss carry-forwards	21,346	21,346
Deferred tax asset, valuation allowance	$ 5,441	$ 5,441	$ 5,735