Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Activities:
Net earnings	$ 140,111	$ 136,200
Adjustments to reconcile net earnings to net cash from operating activities:
Recoveries on accounts receivable	(1,888)	(265)
Deferred income tax expense	2,108	2,853
Stock compensation expense	13,382	11,267
Depreciation and amortization	13,393	13,922
Other, net	189	48
Changes in operating assets and liabilities:
Decrease in accounts receivable	230,477	100,647
Decrease in accounts payable and accrued expenses	(122,283)	(45,844)
Decrease in deferred contract costs	31,259	3,407
Decrease in contract liabilities	(36,725)	(7,401)
Increase in income taxes payable, net	16,993	19,509
Increase (Decrease) in Other Noncurrent Assets and Liabilities, Net	(2,467)	(1,134)
Net cash from operating activities	289,483	235,477
Investing Activities:
Purchase of property and equipment	(9,435)	(13,796)
Other, net	255	(431)
Net cash from investing activities	(9,180)	(14,227)
Financing Activities:
Proceeds from issuance of common stock	27,190	26,397
Repurchases of common stock	(44,334)	(172,360)
Net cash from financing activities	(17,144)	(145,963)
Effect of exchange rate changes on cash and cash equivalents	2,535	6,515
Increase in cash and cash equivalents	265,694	81,802
Cash and cash equivalents at beginning of period	923,735	1,051,099
Cash and cash equivalents at end of period	1,189,429	1,132,901
Taxes Paid:
Income taxes	$ 37,253	$ 38,519