Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net income	$ 261,575	$ 234,461	$ 191,339
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	27,885	26,122	24,157
Amortization	1,389	1,793	1,568
Share-based compensation	13,472	12,874	12,482
Provision (benefit) for doubtful accounts receivable, net of write-offs	(710)	2,286	(154)
Provision for inventory obsolescence, net of write-offs	1,310	1,462	(267)
Provision for deferred income taxes	3,723	4,661	(4,636)
Gains on sales of property and equipment	(85)	(289)	(285)
Equity earnings in unconsolidated investments, net	(262)	(242)	(139)
Net losses (gains) on foreign currency transactions	1,347	560	(171)
Impairment of other non-operating asset	0	0	1,200
Other	3,313	808	166
Changes in operating assets and liabilities, net of effects of acquisitions:
Receivables	(15,691)	(14,371)	(21,903)
Product inventories	(14,165)	(142,170)	(35,783)
Prepaid expenses and other assets	(4,218)	1,018	(4,096)
Accounts payable	16,860	(6,567)	5,077
Accrued expenses and other current liabilities	3,033	(3,750)	6,756
Net cash provided by operating activities	298,776	118,656	175,311
Investing activities
Acquisition of businesses, net of cash acquired	(8,901)	(2,578)	(12,834)
Purchases of property and equipment, net of sale proceeds	(33,362)	(31,580)	(39,390)
Other investments, net	0	0	4
Net cash used in investing activities	(42,263)	(34,158)	(52,220)
Financing activities
Proceeds from revolving line of credit	1,066,529	1,138,195	1,067,868
Payments on revolving line of credit	(1,415,988)	(998,503)	(1,011,977)
Proceeds from asset-backed financing	189,000	198,400	161,600
Payments on asset-backed financing	(182,500)	(189,900)	(145,100)
Proceeds from Term Facility	185,000
Proceeds from short-term borrowings and current portion of long-term debt	30,863	17,127	27,333
Payments on short-term borrowings and current portion of long-term debt	(28,286)	(18,793)	(17,603)
Payments on deferred and contingent acquisition consideration	(312)	(661)	(324)
Purchase of redeemable non-controlling interest	0	0	(2,573)
Payments of deferred financing costs	(406)	(106)	(1,104)
Proceeds from stock issued under share-based compensation plans	18,574	13,569	11,466
Payments of cash dividends	(83,772)	(69,430)	(58,029)
Purchases of treasury stock	(23,188)	(187,469)	(146,006)
Net cash used in financing activities	(244,486)	(97,571)	(114,449)
Effect of exchange rate changes on cash and cash equivalents	198	(509)	(658)
Change in cash and cash equivalents	12,225	(13,582)	7,984
Cash and cash equivalents at beginning of year	16,358	29,940	21,956
Cash and cash equivalents at end of year	$ 28,583	$ 16,358	$ 29,940