Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
OPERATING ACTIVITIES
Net income	$ 78,146	$ 161,788
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	24,393	24,560
Provision for credit losses	5,675	1,774
Stock-based compensation	11,397	17,123
Deferred income taxes	1,622	(364)
Excess tax benefit on stock-based compensation	(3,737)	(4,458)
Other operating activities	788	576
Changes in operating elements (net of acquisitions):
Receivables	(133,142)	117,720
Contract assets	8,713	(5,921)
Prepaid expenses and other	(11,038)	(6,367)
Accounts payable and outstanding checks	98,946	(10,742)
Accrued compensation	(23,879)	(87,259)
Accrued transportation expense	(7,294)	7,331
Accrued income taxes	5,196	39,078
Other accrued liabilities	1,829	1,801
Other assets and liabilities	884	291
Net cash provided by operating activities	58,499	256,931
INVESTING ACTIVITIES
Purchases of property and equipment	(7,841)	(8,619)
Purchases and development of software	(6,862)	(5,246)
Acquisitions, net of cash acquired	(223,617)	(44,143)
Other investing activities	0	8
Net cash used for investing activities	(238,320)	(58,000)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	11,269	19,615
Stock tendered for payment of withholding taxes	(13,795)	(12,042)
Repurchase of common stock	(68,563)	(67,624)
Cash dividends	(69,871)	(69,742)
Proceeds from short-term borrowings	765,600	14,000
Payments on short-term borrowings	(587,600)	(19,000)
Net cash used for financing activities	37,040	(134,793)
Effect of exchange rates on cash	(10,505)	2,720
Net change in cash and cash equivalents	(153,286)	66,858
Cash and cash equivalents, beginning of period	447,858	378,615
Cash and cash equivalents, end of period	$ 294,572	$ 445,473