CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
OPERATING ACTIVITIES
Net income	$ 844,245	$ 506,421	$ 576,968
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	91,259	101,727	100,449
Provision for credit losses	10,649	17,281	5,853
Stock-based compensation	129,977	43,995	39,083
Deferred income taxes	(110,188)	(32,984)	(2,407)
Excess tax benefit on stock-based compensation	(13,101)	(17,581)	(8,492)
Other operating activities	1,915	15,096	(3,830)
Changes in operating elements, net of effects of acquisitions:
Receivables	(1,547,545)	(452,145)	208,312
Contract assets	(257,728)	(65,454)	26,761
Prepaid expenses and other	(43,819)	27,237	(29,871)
Accounts payable and outstanding checks	660,028	180,272	(17,968)
Accrued compensation	63,912	22,547	(40,757)
Accrued transportation expense	189,204	52,380	(18,626)
Accrued income taxes	72,665	51,916	(12,636)
Other accrued liabilities	1,607	26,503	8,937
Other assets and liabilities	1,875	21,980	3,643
Net cash provided by operating activities	94,955	499,191	835,419
INVESTING ACTIVITIES
Purchases of property and equipment	(34,197)	(23,133)	(36,290)
Purchases and development of software	(36,725)	(30,876)	(34,175)
Acquisitions, net of cash acquired	(14,750)	(223,230)	(59,200)
Other investing activities	0	5,525	16,636
Net cash used for investing activities	(85,672)	(271,714)	(113,029)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	70,669	107,657	63,092
Stock tendered for payment of withholding taxes	(26,720)	(17,854)	(15,115)
Repurchase of common stock	(581,756)	(177,514)	(309,444)
Cash dividends	(277,321)	(209,956)	(277,786)
Proceeds from long-term borrowings	300,000	0	1,298,000
Payments on long-term borrowings	(2,048)	0	(1,505,000)
Proceeds from short-term borrowings	3,728,000	1,436,600	185,000
Payments on short-term borrowings	(3,203,251)	(1,579,600)	(90,000)
Net cash provided by (used for) financing activities	7,573	(440,667)	(651,253)
Effect of exchange rates on cash and cash equivalents	(3,239)	9,128	(1,894)
Net change in cash and cash equivalents	13,617	(204,062)	69,243
Cash and cash equivalents, beginning of year	243,796	447,858	378,615
Cash and cash equivalents, end of year	257,413	243,796	447,858
Supplemental cash flow disclosures
Cash paid for income taxes	227,427	93,070	219,029
Cash paid for interest	51,367	47,518	50,854
Accrued share repurchases held in other accrued liabilities	$ 4,293	$ 5,231	$ 0