Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Deferred Compensation Trust	Deferred Compensation Liability
Beginning Balance at Dec. 31, 2014	$ 1,124,255	$ 206	$ 1,325,495	$ 4,887,187	$ (5,088,633)	$ (17,333)	$ 17,333
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	382,927			382,927
Purchase of common stock for treasury	(431,367)				(431,367)
Equity-based compensation	54,091		54,091
Tax benefit from equity benefit plan activity	23,311		23,311
Proceeds from stock options exercised	85,948		85,948
Treasury stock issued upon option exercise and restricted share vesting			(41,050)		41,050
Ending Balance at Dec. 31, 2015	1,239,165	206	1,447,795	5,270,114	(5,478,950)	(17,333)	17,333
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	425,262			425,262
Deferred compensation activity						(42)	42
Purchase of common stock for treasury	(455,351)				(455,351)
Equity-based compensation	43,598		43,598
Tax benefit from equity benefit plan activity	13,661		13,661
Proceeds from stock options exercised	38,106		38,106
Treasury stock issued upon option exercise and restricted share vesting			(27,332)		27,332
Ending Balance at Dec. 31, 2016	1,304,441	206	1,515,828	5,695,376	(5,906,969)	(17,375)	17,375
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative-effect adjustment from adoption of ASU 2016-09, net of tax	609		1,566	(957)
Net income	537,521			537,521
Deferred compensation activity						(8)	8
Purchase of common stock for treasury	(422,166)				(422,166)
Equity-based compensation	44,562		44,562
Tax benefit from equity benefit plan activity	0
Proceeds from stock options exercised	140,525		140,525
Treasury stock issued upon option exercise and restricted share vesting			(58,284)		58,284
Ending Balance at Dec. 31, 2017	$ 1,605,492	$ 206	$ 1,644,197	$ 6,231,940	$ (6,270,851)	$ (17,383)	$ 17,383