Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities:
Net earnings	$ 619,790	$ 490,383	$ 432,533
Adjustments to reconcile net earnings to net cash from operating activities:
Provision for losses on accounts receivable	3,808	5,356	2,607
Deferred income tax (benefit) expense	(12,031)	(43,695)	15,835
Stock compensation expense	56,147	50,908	45,217
Depreciation and amortization	54,019	49,310	46,796
Other	647	(4,382)	(3,540)
Changes in operating assets and liabilities:
Increase in accounts receivable	(214,971)	(184,771)	(102,297)
Increase in accounts payable and accrued expenses	86,036	114,631	102,716
Increase in deferred contract costs	(42,097)	0	0
Increase in contract liabilities	43,928	0	0
(Decrease) increase in income taxes payable, net	(19,691)	16,264	(12,370)
(Increase) decrease in other current assets	(2,781)	(5,365)	1,988
Net cash from operating activities	572,804	488,639	529,485
Investing Activities:
Purchase of short-term investments	(27)	(12)	(54)
Proceeds from maturities of short-term investments	59	12	17
Purchase of property and equipment	(47,474)	(95,016)	(59,316)
Proceeds from sale of property and equipment	215	84,405	229
Other, net	(1,172)	(1,074)	5,928
Net cash from investing activities	(48,399)	(11,685)	(53,196)
Financing Activities:
Proceeds from issuance of common stock	182,732	205,092	185,313
Repurchases of common stock	(647,898)	(478,258)	(337,658)
Dividends paid	(156,840)	(150,495)	(145,123)
Payments Related to Tax Withholding for Share-based Compensation	(3,248)	0	0
Payments to Noncontrolling Interests	(688)	0	0
Distributions to noncontrolling interest	(1,796)	(904)	(1,335)
Net cash from financing activities	(627,738)	(424,565)	(298,803)
Effect of exchange rate changes on cash and cash equivalents	(24,031)	24,275	(10,847)
(Decrease) increase in cash and cash equivalents	(127,364)	76,664	166,639
Cash and cash equivalents at beginning of year	1,051,099	974,435	807,796
Cash and cash equivalents at end of year	923,735	1,051,099	974,435
Supplemental Cash Flow Information:
Cash paid for income taxes	$ 239,255	$ 249,704	$ 254,312