Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 357,796	$ 305,928	$ 252,195
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	192	557	1,700
Depreciation expense	53,273	49,124	47,072
Stock-based compensation expense—restricted stock and stock units	41,292	40,821	38,867
Excess tax benefits from stock-based compensation	(8,762)	(7,174)	(8,103)
Deferred income taxes	(8,579)	(3,643)	(13,259)
Provision for doubtful accounts	12,005	9,825	7,467
Changes in assets and liabilities, net of effects of acquisitions:
Increase in accounts receivable	(75,745)	(134,917)	(47,699)
Increase in accounts payable, accrued expenses, accrued payroll and benefit costs	60,232	71,740	38,356
Increase (decrease) in income taxes payable	19,948	16,359	(11,927)
Change in other assets, net of change in other liabilities	(13,416)	(7,922)	4,548
Net cash flows provided by operating activities	438,236	340,698	309,217
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired	(14,668)	0	0
Capital expenditures	(75,057)	(62,830)	(53,725)
Payments to trusts for employee deferred compensation plans	(28,225)	(25,811)	(44,052)
Net cash flows used in investing activities	(117,950)	(88,641)	(97,777)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(271,138)	(153,821)	(167,975)
Cash dividends paid	(107,561)	(97,604)	(89,187)
Decrease in notes payable and other indebtedness	(140)	(128)	(4,496)
Excess tax benefits from stock-based compensation	8,762	7,174	8,103
Proceeds from exercises of stock options	1,529	14,324	33,285
Net cash flows used in financing activities	(368,548)	(230,055)	(220,270)
Effect of exchange rate changes on cash and cash equivalents	(14,280)	(10,647)	(3,041)
Net (decrease) increase in cash and cash equivalents	(62,542)	11,355	(11,871)
Cash and cash equivalents at beginning of period	287,119	275,764	287,635
Cash and cash equivalents at end of period	224,577	287,119	275,764
Cash paid during the year for:
Interest	285	330	315
Income taxes, net of refunds	212,668	178,375	168,407
Non-cash items:
Stock repurchases awaiting settlement	$ 11,935	$ 30,152	$ 0