Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
OPERATING ACTIVITIES
Net income	$ 222,085	$ 330,968
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	50,151	49,642
Provision for credit losses	9,374	3,224
Stock-based compensation	22,351	31,807
Deferred income taxes	(729)	(5,322)
Excess tax benefit on stock-based compensation	(11,999)	(5,353)
Other operating activities	12,341	961
Changes in operating elements, net of acquisitions:
Receivables	(48,937)	89,175
Contract assets	(22,451)	(19,380)
Prepaid expenses and other	8,744	(16,404)
Accounts payable and outstanding checks	220,276	37,378
Accrued compensation	12,312	(60,976)
Accrued transportation expense	20,284	19,149
Accrued income taxes	14,423	(3,051)
Other accrued liabilities	(6,345)	4,166
Other assets and liabilities	3,763	542
Net cash provided by operating activities	505,643	456,526
INVESTING ACTIVITIES
Purchases of property and equipment	(11,621)	(16,774)
Purchases and development of software	(13,418)	(14,790)
Acquisitions, net of cash acquired	(223,230)	(58,379)
Other investing activities	5,525	8
Net cash used for investing activities	(242,744)	(89,935)
FINANCING ACTIVITIES
Proceeds from stock issued for employee benefit plans	20,295	27,952
Stock tendered for payment of withholding taxes	(14,966)	(12,680)
Repurchase of common stock	(68,563)	(173,622)
Cash dividends	(137,104)	(139,010)
Proceeds from long-term borrowings	0	473,000
Payments on long-term borrowings	0	(561,000)
Proceeds from short-term borrowings	979,600	14,000
Payments on short-term borrowings	(1,122,600)	(19,000)
Net cash used for financing activities	(343,338)	(390,360)
Effect of exchange rates on cash	(5,183)	461
Net change in cash and cash equivalents	(85,622)	(23,308)
Cash and cash equivalents, beginning of period	447,858	378,615
Cash and cash equivalents, end of period	$ 362,236	$ 355,307