Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net income	$ 219,923	$ 205,544	$ 98,177
Adjustments to reconcile net income to net cash provided by operations:
Depreciation and amortization	45,205	38,863	35,625
Stock option expense	14,831	12,611	10,485
Accrued litigation settlement	6,000
Noncash portion of long-term incentive compensation	5,740	5,405	3,774
Provision/(benefit) for deferred income taxes (Note 11)	(2,770)	5,187	2,407
Noncash directors' compensation	767	766	766
Loss on sale of transportation equipment (Note 19)	2,266		5,266
Amortization of debt issuance costs	306	441	516
Amortization of restricted stock awards		446	1,231
Provision for uncollectible accounts receivable			17,306
Changes in operating assets and liabilities:
Decrease/(increase) in accounts receivable	(19,247)	(5,570)	1,072
Decrease/(increase) in inventories	(1,757)	(351)	421
Increase in prepaid expenses	(3,491)	(2,665)	(2,987)
Increase in accounts payable and other current liabilities	28,417	8,935	12,890
Net change in lease assets and liabilities	3,108
Change in current income taxes	161	18,898	(26,104)
Increase in other assets	(11,963)	(5,544)	(8,330)
Increase in other liabilities	12,354	3,451	8,561
Other sources	1,399	721	1,419
Net cash provided by operating activities	301,249	287,138	162,495
Cash Flows from Investing Activities
Business combinations, net of cash acquired (Note 7)	(138,010)	(53,177)	(4,725)
Capital expenditures	(53,022)	(52,872)	(64,300)
Other sources	272	824	1,417
Net cash used by investing activities	(190,760)	(105,225)	(67,608)
Cash Flows from Financing Activities
Proceeds from revolving line of credit	482,900	469,550	212,350
Payments on revolving line of credit	(482,100)	(406,550)	(211,150)
Purchases of treasury stock	(92,631)	(158,884)	(94,640)
Proceeds from exercise of stock options (Note 4)	34,380	32,412	27,092
Capital stock surrendered to pay taxes on stock-based compensation	(28,474)	(27,548)	(14,223)
Dividends paid	(19,788)	(18,662)	(17,371)
Change in cash overdrafts payable	(3,927)	(1,531)	6,700
Payments on other long-term debt		(75,000)	(8,750)
Debt issuance costs		(1,052)
Other sources/(uses)	478	(938)	916
Net cash used by financing activities	(109,162)	(188,203)	(99,076)
Increase/(decrease) in cash and cash equivalents	1,327	(6,290)	(4,189)
Cash and cash equivalents at beginning of year	4,831	11,121	15,310
Cash and cash equivalents at end of year	$ 6,158	$ 4,831	$ 11,121