CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income	$ 664,505	$ 504,893	$ 513,384
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	96,729	92,977	74,669
Provision for doubtful accounts	15,634	13,489	5,136
Stock-based compensation	87,791	41,805	37,565
Deferred income taxes	(15,315)	(28,096)	15,009
Excess tax benefit on stock-based compensation	(10,388)	(13,657)
Excess tax benefit on stock-based compensation			(18,462)
Other operating activities	1,815	4,491	1,907
Changes in operating elements, net of effects of acquisitions:
Receivables	(190,048)	(364,181)	(173,211)
Contract assets	(11,871)	0	0
Prepaid expenses and other	16,029	(9,173)	(6,378)
Other non-current assets	1,370	(19,099)	(3,934)
Accounts payable and outstanding checks	36,083	144,041	115,917
Accrued compensation	47,011	7,209	(47,570)
Accrued transportation expense	25,175	0	0
Accrued income taxes	21,176	18,817	19,921
Other accrued liabilities	7,200	(9,515)	(4,545)
Net cash provided by operating activities	792,896	384,001	529,408
INVESTING ACTIVITIES
Purchases of property and equipment	(45,000)	(40,122)	(73,452)
Purchases and development of software	(18,871)	(17,823)	(17,985)
Acquisitions, net of cash acquired	(5,315)	(49,068)	(220,203)
Other investing activities	(3,622)	(521)	(1,348)
Net cash used for investing activities	(72,808)	(107,534)	(312,988)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	51,285	38,130	19,271
Stock tendered for payment of withholding taxes	(21,264)	(21,557)	(36,678)
Repurchase of common stock	(300,991)	(185,485)	(172,925)
Cash dividends	(265,219)	(258,222)	(245,430)
Excess tax benefit on stock-based compensation	0	0	18,462
Proceeds from long-term borrowings	591,012	250,000	0
Proceeds from short-term borrowings	2,674,000	8,784,000	6,600,000
Payments on short-term borrowings	(3,384,000)	(8,809,000)	(6,310,000)
Net cash used for financing activities	(655,177)	(202,134)	(127,300)
Effect of exchange rates on cash	(20,186)	11,891	(9,683)
Net change in cash and cash equivalents	44,725	86,224	79,437
Cash and cash equivalents, beginning of year	333,890	247,666	168,229
Cash and cash equivalents, end of year	378,615	333,890	247,666
Supplemental cash flow disclosures
Cash paid for income taxes	215,644	262,861	269,187
Cash paid for interest	47,544	37,871	28,908
Accrued share repurchases held in other accrued liabilities	$ 3,000	$ 500	$ 5,988