Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income	$ 191,339	$ 148,603	$ 128,224
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	24,157	20,338	16,373
Amortization	1,568	1,639	1,015
Share-based compensation	12,482	9,902	9,543
Excess tax benefits from share-based compensation	0	(7,370)	(7,706)
Provision for doubtful accounts receivable, net of write-offs	(154)	(155)	197
Provision for inventory obsolescence, net of write-offs	(267)	(448)	576
Provision for deferred income taxes	(4,636)	3,749	4,198
(Gains) losses on sale of property and equipment	(285)	(320)	230
Equity earnings in unconsolidated investments, net	(139)	(156)	(211)
Net (gains) losses on foreign currency transactions	(171)	679	774
Impairment of goodwill and other non-operating assets	1,200	4,113	500
Other	166	923	(869)
Changes in operating assets and liabilities, net of effects of acquisitions:
Receivables	(21,903)	(5,666)	(16,656)
Product inventories	(35,783)	(8,050)	(10,848)
Prepaid expenses and other assets	(4,096)	(3,077)	(434)
Accounts payable	5,077	(17,896)	9,956
Accrued expenses and other current liabilities	6,756	18,570	11,188
Net cash provided by operating activities	175,311	165,378	146,050
Investing activities
Acquisition of businesses, net of cash acquired	(12,834)	(19,730)	(4,483)
Purchases of property and equipment, net of sale proceeds	(39,390)	(34,352)	(29,095)
Payments to fund credit agreement	0	(5,322)	(8,860)
Collections from credit agreement	0	3,737	4,557
Other investments, net	4	24	88
Net cash used in investing activities	(52,220)	(55,643)	(37,793)
Financing activities
Proceeds from revolving line of credit	1,067,868	1,154,090	911,712
Payments on revolving line of credit	(1,011,977)	(1,072,557)	(890,406)
Proceeds from asset-backed financing	161,600	155,000	143,400
Payments on asset-backed financing	(145,100)	(126,500)	(156,000)
Proceeds from short-term borrowings and current portion of long-term debt	27,333	18,442	8,119
Payments on short-term borrowings and current portion of long-term debt	(17,603)	(19,037)	(7,948)
Payments on deferred and contingent acquisition consideration	(324)	0	0
Purchase of redeemable non-controlling interest	(2,573)	0	0
Payments of deferred financing costs	(1,104)	(69)	(320)
Excess tax benefits from share-based compensation	0	7,370	7,706
Proceeds from stock issued under share-based compensation plans	11,466	11,752	18,269
Payments of cash dividends	(58,029)	(49,749)	(43,117)
Purchases of treasury stock	(146,006)	(178,414)	(99,219)
Net cash used in financing activities	(114,449)	(99,672)	(107,804)
Effect of exchange rate changes on cash and cash equivalents	(658)	(1,344)	(2,046)
Change in cash and cash equivalents	7,984	8,719	(1,593)
Cash and cash equivalents at beginning of year	21,956	13,237	14,830
Cash and cash equivalents at end of year	$ 29,940	$ 21,956	$ 13,237