CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES
Net income	$ 576,968	$ 664,505	$ 504,893
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	100,449	96,729	92,977
Provision for doubtful accounts	5,853	15,634	13,489
Stock-based compensation	39,083	87,791	41,805
Deferred income taxes	(2,407)	(15,315)	(28,096)
Excess tax benefit on stock-based compensation	(8,492)	(10,388)	(13,657)
Other operating activities	(3,830)	1,815	4,491
Changes in operating elements, net of effects of acquisitions:
Receivables	208,312	(190,048)	(364,181)
Contract assets	26,761	(11,871)	0
Prepaid expenses and other	(29,871)	16,029	(9,173)
Accounts payable and outstanding checks	(17,968)	36,083	144,041
Accrued compensation	(40,757)	47,011	7,209
Accrued transportation expense	(18,626)	25,175	0
Accrued income taxes	(12,636)	21,176	18,817
Other accrued liabilities	8,937	7,200	(9,515)
Other assets and liabilities	3,643	1,370	(19,099)
Net cash provided by operating activities	835,419	792,896	384,001
INVESTING ACTIVITIES
Purchases of property and equipment	(36,290)	(45,000)	(40,122)
Purchases and development of software	(34,175)	(18,871)	(17,823)
Acquisitions, net of cash acquired	(59,200)	(5,315)	(49,068)
Other investing activities	16,636	(3,622)	(521)
Net cash used for investing activities	(113,029)	(72,808)	(107,534)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	63,092	51,285	38,130
Stock tendered for payment of withholding taxes	(15,115)	(21,264)	(21,557)
Repurchase of common stock	(309,444)	(300,991)	(185,485)
Cash dividends	(277,786)	(265,219)	(258,222)
Proceeds from long-term borrowings	1,298,000	591,012	250,000
Payments on long-term borrowings	(1,505,000)	0	0
Proceeds from short-term borrowings	185,000	2,674,000	8,784,000
Payments on short-term borrowings	(90,000)	(3,384,000)	(8,809,000)
Net cash used for financing activities	(651,253)	(655,177)	(202,134)
Effect of exchange rates on cash and cash equivalents	(1,894)	(20,186)	11,891
Net change in cash and cash equivalents	69,243	44,725	86,224
Cash and cash equivalents, beginning of year	378,615	333,890	247,666
Cash and cash equivalents, end of year	447,858	378,615	333,890
Supplemental cash flow disclosures
Cash paid for income taxes	219,029	215,644	262,861
Cash paid for interest	50,854	47,544	37,871
Accrued share repurchases held in other accrued liabilities	$ 0	$ 3,000	$ 500