CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income	$ 504,893	$ 513,384	$ 509,699
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	92,977	74,669	66,409
Provision for doubtful accounts	13,489	5,136	11,538
Stock-based compensation	41,805	37,565	57,661
Deferred income taxes	(28,096)	15,009	(17,095)
Excess tax benefit on stock-based compensation	(13,657)	(18,462)	(8,548)
Other	4,491	1,907	7,409
Changes in operating elements, net of effects of acquisitions:
Receivables	(364,181)	(173,211)	107,560
Prepaid expenses and other	(9,173)	(6,378)	(228)
Other non-current assets	(19,099)	(3,934)	741
Accounts payable and outstanding checks	144,041	115,917	(53,272)
Accrued compensation	7,209	(47,570)	18,580
Accrued income taxes	18,817	19,921	13,726
Other accrued liabilities	(9,515)	(4,545)	4,156
Net cash provided by operating activities	384,001	529,408	718,336
INVESTING ACTIVITIES
Purchases of property and equipment	(40,122)	(73,452)	(28,115)
Purchases and development of software	(17,823)	(17,985)	(16,527)
Acquisitions, net of cash acquired	(49,068)	(220,203)	(369,833)
Restricted cash	0	0	359,388
Other	(521)	(1,348)	641
Net cash used for investing activities	(107,534)	(312,988)	(54,446)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	38,130	19,271	15,557
Stock tendered for payment of withholding taxes	(21,557)	(36,678)	(11,368)
Repurchase of common stock	(185,485)	(172,925)	(229,863)
Cash dividends	(258,222)	(245,430)	(235,615)
Excess tax benefit on stock-based compensation	0	18,462	8,548
Proceeds from long-term borrowings	250,000	0	0
Proceeds from short-term borrowings	8,784,000	6,600,000	6,833,000
Payments on short-term borrowings	(8,809,000)	(6,310,000)	(6,988,000)
Net cash used for financing activities	(202,134)	(127,300)	(607,741)
Effect of exchange rates on cash	11,891	(9,683)	(16,860)
Net change in cash and cash equivalents	86,224	79,437	39,289
Cash and cash equivalents, beginning of year	247,666	168,229	128,940
Cash and cash equivalents, end of year	333,890	247,666	168,229
Supplemental cash flow disclosures
Cash paid for income taxes	262,861	269,187	311,800
Cash paid for interest	37,871	28,908	28,537
Accrued share repurchases held in other accrued liabilities	$ 500	$ 5,988	$ 2,250