Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Activities:
Net earnings	$ 163,067	$ 120,606	$ 440,213	$ 322,928
Adjustments to reconcile net earnings to net cash from operating activities:
Provision for losses on accounts receivable	1,969	1,741	2,232	3,187
Deferred income tax expense	(23,610)	(28,854)	(17,522)	(16,000)
Stock compensation expense	13,902	11,210	43,171	39,036
Depreciation and amortization	13,335	12,272	40,833	36,241
Other, net	666	377	770	(148)
Changes in operating assets and liabilities:
Increase in accounts receivable	(210,092)	(126,102)	(156,321)	(123,790)
Increase in accounts payable and accrued expenses	115,629	61,833	127,860	96,132
Increase in deferred contract costs	(34,623)	0	(51,235)	0
Increase in contract liabilities	36,256	0	49,149	0
Increase (decrease) in income taxes payable, net	9,292	38,149	(9,258)	10,814
Decrease (increase) in other current assets	930	(5,872)	862	(6,147)
Net cash from operating activities	86,721	85,360	470,754	362,253
Investing Activities:
Purchase of property and equipment	(11,733)	(34,462)	(37,642)	(67,603)
Other, net	1,109	(261)	(886)	(892)
Net cash from investing activities	(10,624)	(34,723)	(38,528)	(68,495)
Financing Activities:
Proceeds from issuance of common stock	47,806	65,915	169,566	162,781
Repurchases of common stock	(147,828)	(202,776)	(555,760)	(340,736)
Dividends paid	0	0	79,180	75,726
Payments for taxes related to net share settlement of equity awards	0	0	(3,215)	0
Distributions to noncontrolling interest	(633)	0	(633)	0
Purchase of noncontrolling interest	(613)	0	(613)	0
Net cash from financing activities	(101,268)	(136,861)	(469,835)	(253,681)
Effect of exchange rate changes on cash and cash equivalents	(5,071)	4,758	(22,878)	18,932
(Decrease) increase in cash and cash equivalents	(30,242)	(81,466)	(60,487)	59,009
Cash and cash equivalents at beginning of period	1,020,854	1,114,910	1,051,099	974,435
Cash and cash equivalents at end of period	990,612	1,033,444	990,612	1,033,444
Taxes Paid:
Income taxes	$ 59,313	$ 58,257	$ 183,444	$ 190,911