Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Activities:
Net earnings	$ 140,946	$ 108,755	$ 277,146	$ 202,322
Adjustments to reconcile net earnings to net cash from operating activities:
Provision for losses on accounts receivable	528	515	263	1,446
Deferred income tax expense	3,235	7,261	6,088	12,854
Stock compensation expense	18,002	17,203	29,269	27,826
Depreciation and amortization	13,576	12,042	27,498	23,969
Other, net	56	(174)	104	(525)
Changes in operating assets and liabilities:
(Increase) decrease in accounts receivable	(46,876)	(73,142)	53,771	2,312
Decrease in accounts payable and accrued expenses	58,075	52,623	12,231	34,299
Increase in deferred contract costs	(20,019)	0	(16,612)	0
Increase in contract liabilities	20,294	0	12,893	0
Decrease in income taxes payable, net	(38,059)	(47,159)	(18,550)	(27,335)
(Increase) decrease in other current assets	(1,202)	3,290	(68)	(275)
Net cash from operating activities	148,556	81,214	384,033	276,893
Investing Activities:
Purchase of property and equipment	(12,113)	(20,380)	(25,909)	(33,141)
Other, net	(1,564)	40	(1,995)	(631)
Net cash from investing activities	(13,677)	(20,340)	(27,904)	(33,772)
Financing Activities:
Proceeds from issuance of common stock	95,363	51,501	121,760	96,866
Repurchases of common stock	(235,572)	(84,052)	(407,932)	(137,960)
Dividends paid	79,180	75,726	79,180	75,726
Payments for taxes related to net share settlement of equity awards	(3,215)	0	(3,215)	0
Net cash from financing activities	(222,604)	(108,277)	(368,567)	(116,820)
Effect of exchange rate changes on cash and cash equivalents	(24,322)	6,270	(17,807)	14,174
(Decrease) increase in cash and cash equivalents	(112,047)	(41,133)	(30,245)	140,475
Cash and cash equivalents at beginning of period	1,132,901	1,156,043	1,051,099	974,435
Cash and cash equivalents at end of period	1,020,854	1,114,910	1,020,854	1,114,910
Taxes Paid:
Income taxes	$ 85,612	$ 103,508	$ 124,131	$ 132,654