Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Activities:
Net earnings	$ 136,200	$ 93,567
Adjustments to reconcile net earnings to net cash from operating activities:
(Recoveries) provision for losses on accounts receivable	(265)	931
Deferred income tax expense	2,853	5,593
Stock compensation expense	11,267	10,623
Depreciation and amortization	13,922	11,927
Other, net	48	(351)
Changes in operating assets and liabilities:
Decrease in accounts receivable	100,647	75,454
Decrease in accounts payable and accrued expenses	(45,844)	(18,324)
Decrease in deferred contract costs	3,407	0
Decrease in contract liabilities	(7,401)	0
Increase in income taxes payable, net	19,509	19,824
Decrease (increase) in other current assets	1,134	(3,565)
Net cash from operating activities	235,477	195,679
Investing Activities:
Purchase of property and equipment	(13,796)	(12,761)
Other, net	(431)	(671)
Net cash from investing activities	(14,227)	(13,432)
Financing Activities:
Proceeds from issuance of common stock	26,397	45,365
Repurchases of common stock	(172,360)	(53,908)
Net cash from financing activities	(145,963)	(8,543)
Effect of exchange rate changes on cash and cash equivalents	6,515	7,904
Increase in cash and cash equivalents	81,802	181,608
Cash and cash equivalents at beginning of period	1,051,099	974,435
Cash and cash equivalents at end of period	1,132,901	1,156,043
Taxes Paid:
Income taxes	$ 38,519	$ 29,146