Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	COMMON STOCK	CAPITAL SURPLUS	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)	RETAINED EARNINGS
Balance at beginning of period (in shares) at Dec. 31, 2017		124,261
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		525
Repurchases of common stock (in shares)		(2,341)
Balance at end of period (in shares) at Jun. 30, 2018		122,445
Balance at beginning of period at Dec. 31, 2017		$ 124	$ 1,064,601	$ 3,507	$ 37,033
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Repurchases of common stock		(3)			(144,482)
Stock-based compensation expense			21,945
Foreign currency translation adjustments, net of tax	$ (9,916)			(9,916)
Net income	205,482				205,482
Cash dividends ($.31 per share, $.28 per share, $.62 per share, and $.56 per share)					(68,793)
Balance at end of period at Jun. 30, 2018		$ 122	1,086,545	(6,409)	29,240
Balance at beginning of period (in shares) at Mar. 31, 2018		123,563
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		12
Repurchases of common stock (in shares)		(1,130)
Balance at end of period (in shares) at Jun. 30, 2018		122,445
Balance at beginning of period at Mar. 31, 2018		$ 124	1,075,156	9,015	30,502
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		0	0
Repurchases of common stock		(2)			(75,994)
Stock-based compensation expense			11,389
Foreign currency translation adjustments, net of tax	(15,424)			(15,424)
Net income	109,315				109,315
Cash dividends ($.31 per share, $.28 per share, $.62 per share, and $.56 per share)					(34,583)
Balance at end of period at Jun. 30, 2018		$ 122	1,086,545	(6,409)	29,240
Balance at beginning of period (in shares) at Dec. 31, 2018		119,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		552
Repurchases of common stock (in shares)		(2,069)
Balance at end of period (in shares) at Jun. 30, 2019		117,561
Balance at beginning of period at Dec. 31, 2018	1,063,198	$ 119	1,079,188	(16,109)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Repurchases of common stock		(2)			(127,947)
Stock-based compensation expense			22,914
Foreign currency translation adjustments, net of tax	249			249
Net income	224,410				224,410
Cash dividends ($.31 per share, $.28 per share, $.62 per share, and $.56 per share)					(73,595)
Balance at end of period at Jun. 30, 2019	1,109,227	$ 118	1,102,101	(15,860)	22,868
Balance at beginning of period (in shares) at Mar. 31, 2019		118,321
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock (in shares)		271
Repurchases of common stock (in shares)		(1,031)
Balance at end of period (in shares) at Jun. 30, 2019		117,561
Balance at beginning of period at Mar. 31, 2019		$ 118	1,090,432	(18,006)	4,485
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net issuances of restricted stock		1	(1)
Repurchases of common stock		(1)			(59,632)
Stock-based compensation expense			11,670
Foreign currency translation adjustments, net of tax				2,146
Net income	114,612				114,612
Cash dividends ($.31 per share, $.28 per share, $.62 per share, and $.56 per share)					(36,597)
Balance at end of period at Jun. 30, 2019	$ 1,109,227	$ 118	$ 1,102,101	$ (15,860)	$ 22,868