Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014	$ 908,924	$ 323,596	$ 587,862	$ (2,534)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,795		86,795
Other comprehensive loss, net of tax	2,940			2,940
Common stock transactions:
Common shares purchased and retired	(20,647)	(20,647)
Common shares issued	0
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	3,369	3,369
Tax benefit of stock-based compensation	1,443	1,443
Stock-based compensation expense	3,959	3,959
Cash dividends declared:
Common (per share)	(36,290)		(36,290)
Ending Balance at Dec. 31, 2015	950,493	311,720	638,367	406
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	95,636		95,636
Other comprehensive loss, net of tax	(8,534)			(8,534)
Common stock transactions:
Common shares purchased and retired	(26,854)	(26,854)
Common shares issued	0
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	4,683	4,683
Tax benefit of stock-based compensation	2,146	2,146
Stock-based compensation expense	4,376	4,376
Cash dividends declared:
Common (per share)	(39,353)		(39,353)
Ending Balance at Dec. 31, 2016	982,593	296,071	694,650	(8,128)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106,521		106,521
Other comprehensive loss, net of tax	(3,835)			(3,835)
Common stock transactions:
Common shares purchased and retired	(1,348)	(1,348)
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	2,431	2,431
Stock-based compensation expense	3,868	3,868
Cash dividends declared:
Common (per share)	(48,583)		(48,583)
Ending Balance at Dec. 31, 2017	1,427,616	686,991	$ 752,588	$ (11,963)
Common stock transactions:
Common Stock, Value, Issued	$ 385,969	$ 385,969