Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities
Net income	$ 319,466	$ 219,923	$ 205,544
Adjustments to reconcile net income to net cash provided by operations:
Depreciation and amortization	56,583	45,205	38,863
Stock option expense	18,422	14,831	12,611
Deferred payroll taxes	36,350
Noncash portion of long-term incentive compensation	7,208	5,740	5,405
Accrued litigation settlement	2,684	6,000
Provision/(benefit) for deferred income taxes (Note 11)	1,433	(2,770)	5,187
Noncash directors' compensation	1,171	767	766
Amortization of debt issuance costs	306	306	441
Loss on sale of transportation equipment (Note 19)		2,266
Amortization of restricted stock awards			446
Changes in operating assets and liabilities:
Decrease/(increase) in accounts receivable	12,773	(19,247)	(5,570)
Decrease/(increase) in inventories	367	(1,757)	(351)
Increase in prepaid expenses	(3,027)	(3,491)	(2,665)
Increase in accounts payable and other current liabilities	19,096	28,417	8,935
Net change in lease assets and liabilities	1,206	3,108
Change in current income taxes	13,525	161	18,898
Increase in other assets	(11,834)	(11,963)	(5,544)
Increase in other liabilities	12,323	12,354	3,451
Other sources	1,237	1,399	721
Net cash provided by operating activities	489,289	301,249	287,138
Cash Flows from Investing Activities
Capital expenditures	(58,831)	(53,022)	(52,872)
Business combinations, net of cash acquired (Note 7)	(3,600)	(138,010)	(53,177)
Other sources	871	272	824
Net cash used by investing activities	(61,560)	(190,760)	(105,225)
Cash Flows from Financing Activities
Payments on revolving line of credit	(264,900)	(482,100)	(406,550)
Proceeds from revolving line of credit	174,900	482,900	469,550
Purchases of treasury stock	(175,594)	(92,631)	(158,884)
Proceeds from exercise of stock options (Note 4)	50,382	34,380	32,412
Capital stock surrendered to pay taxes on stock-based compensation	(25,328)	(28,474)	(27,548)
Dividends paid	(21,079)	(19,788)	(18,662)
Change in cash overdraft payable	(9,849)	(3,927)	(1,531)
Payments on other long-term debt			(75,000)
Other sources/(uses)	256	478	(1,990)
Net cash used by financing activities	(271,212)	(109,162)	(188,203)
Increase/(decrease) in cash and cash equivalents	156,517	1,327	(6,290)
Cash and cash equivalents at beginning of year	6,158	4,831	11,121
Cash and cash equivalents at end of year	$ 162,675	$ 6,158	$ 4,831