Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities:
Net earnings	$ 698,214	$ 592,016	$ 619,790
Adjustments to reconcile net earnings to net cash from operating activities:
Provisions for losses (recoveries) on accounts receivable	5,584	(1)	3,808
Deferred income tax expense (benefit)	8,371	4,482	(12,031)
Stock compensation expense	62,498	61,543	56,147
Depreciation and amortization	56,959	50,950	54,019
Other, net	3,960	941	647
Changes in operating assets and liabilities:
(Increase) decrease in accounts receivable	(647,193)	265,919	(214,971)
Increase (decrease) in accounts payable and accrued expenses	430,495	(181,987)	86,036
(Increase) decrease in deferred contract costs	(189,447)	28,811	(42,097)
Increase (decrease) in contract liabilities	217,699	(37,097)	43,928
Increase (decrease) in income taxes payable, net	8,502	(18,472)	(19,691)
(Increase) decrease in other, net	(630)	4,830	(2,781)
Net cash from operating activities	655,012	771,935	572,804
Investing Activities:
Purchase of property and equipment	(47,543)	(47,022)	(47,474)
Other, net	1,516	1,007	(925)
Net cash from investing activities	(46,027)	(46,015)	(48,399)
Financing Activities:
Proceeds from issuance of common stock	186,345	148,245	182,732
Repurchases of common stock	(332,387)	(389,060)	(647,898)
Dividends Paid	(174,929)	(170,553)	(156,840)
Payments for taxes related to net share settlement of equity awards	(10,566)	(6,674)	(3,248)
Purchase of noncontrolling interest			(688)
Distributions to noncontrolling interest			(1,796)
Net cash from financing activities	(331,537)	(418,042)	(627,738)
Effect of exchange rate changes on cash and cash equivalents	19,852	(1,122)	(24,031)
Change in cash and cash equivalents	297,300	306,756	(127,364)
Cash and cash equivalents at beginning of period	1,230,491	923,735	1,051,099
Cash and cash equivalents at end of period	1,527,791	1,230,491	923,735
Supplemental Cash Flow Information:
Cash paid for income taxes	$ 239,849	$ 222,083	$ 239,255