Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income	$ 234,461	$ 191,339	$ 148,603
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	26,122	24,157	20,338
Amortization	1,793	1,568	1,639
Share-based compensation	12,874	12,482	9,902
Excess tax benefits from share-based compensation	0	0	(7,370)
Provision for doubtful accounts receivable, net of write-offs	2,286	(154)	(155)
Provision for inventory obsolescence, net of write-offs	1,462	(267)	(448)
Provision for deferred income taxes	4,661	(4,636)	3,749
(Gains) losses on sale of property and equipment	(289)	(285)	(320)
Equity earnings in unconsolidated investments, net	(242)	(139)	(156)
Net (gains) losses on foreign currency transactions	560	(171)	679
Impairment of goodwill and other non-operating assets	0	1,200	4,113
Other	808	166	923
Changes in operating assets and liabilities, net of effects of acquisitions:
Receivables	(14,371)	(21,903)	(5,666)
Product inventories	(142,170)	(35,783)	(8,050)
Prepaid expenses and other assets	1,018	(4,096)	(3,077)
Accounts payable	(6,567)	5,077	(17,896)
Accrued expenses and other current liabilities	(3,750)	6,756	18,570
Net cash provided by operating activities	118,656	175,311	165,378
Investing activities
Acquisition of businesses, net of cash acquired	(2,578)	(12,834)	(19,730)
Purchases of property and equipment, net of sale proceeds	(31,580)	(39,390)	(34,352)
Payments to fund credit agreement	0	0	(5,322)
Collections from credit agreement	0	0	3,737
Other investments, net	0	4	24
Net cash used in investing activities	(34,158)	(52,220)	(55,643)
Financing activities
Proceeds from revolving line of credit	1,138,195	1,067,868	1,154,090
Payments on revolving line of credit	(998,503)	(1,011,977)	(1,072,557)
Proceeds from asset-backed financing	198,400	161,600	155,000
Payments on asset-backed financing	(189,900)	(145,100)	(126,500)
Proceeds from short-term borrowings and current portion of long-term debt	17,127	27,333	18,442
Payments on short-term borrowings and current portion of long-term debt	(18,793)	(17,603)	(19,037)
Payments on deferred and contingent acquisition consideration	(661)	(324)	0
Purchase of redeemable non-controlling interest	0	(2,573)	0
Payments of deferred financing costs	(106)	(1,104)	(69)
Excess tax benefits from share-based compensation	0	0	7,370
Proceeds from stock issued under share-based compensation plans	13,569	11,466	11,752
Payments of cash dividends	(69,430)	(58,029)	(49,749)
Purchases of treasury stock	(187,469)	(146,006)	(178,414)
Net cash used in financing activities	(97,571)	(114,449)	(99,672)
Effect of exchange rate changes on cash and cash equivalents	(509)	(658)	(1,344)
Change in cash and cash equivalents	(13,582)	7,984	8,719
Cash and cash equivalents at beginning of year	29,940	21,956	13,237
Cash and cash equivalents at end of year	$ 16,358	$ 29,940	$ 21,956