CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Beginning balance (in shares) at Dec. 31, 2014		65,303
Beginning balance at Dec. 31, 2014	$ 104,138	$ 7	$ 94,690	$ 9,441
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41,979			41,979
Dividends declared	(36,456)			(36,456)
Dividend equivalents on unvested performance-based restricted stock awards	(159)			(159)
Dividend equivalents cancelled upon forfeiture of performance-based restricted stock awards	82		17	65
Restricted stock vestings (in shares)		365
Restricted stock vestings	73	$ 0	73
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings (in shares)		(106)
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings	(1,838)	$ 0	(1,838)
Stock-based compensation	6,441		6,441
Excess tax deductions relating to stock-based compensation	413		413
Shares issuable in lieu of dividend payments on performance-based restricted stock awards	2		2
Issuance of common stock under employee stock purchase plan (in shares)		75
Issuance of common stock under employee stock purchase plan	1,131	$ 0	1,131
Ending balance (in shares) at Dec. 31, 2015		65,637
Ending balance at Dec. 31, 2015	115,806	$ 7	100,929	14,870
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55,833			55,833
Dividends declared	(43,301)			(43,301)
Dividend equivalents on unvested performance-based restricted stock awards	(202)			(202)
Dividend equivalents cancelled upon forfeiture of performance-based restricted stock awards	27		0	27
Restricted stock vestings (in shares)		390
Restricted stock vestings	136	$ 0	136
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings (in shares)		(120)
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings	(2,137)	$ 0	(2,137)
Stock-based compensation	5,997		5,997
Excess tax deductions relating to stock-based compensation	590		590
Shares issuable in lieu of dividend payments on performance-based restricted stock awards	40		40
Issuance of common stock under employee stock purchase plan (in shares)		75
Issuance of common stock under employee stock purchase plan	$ 1,114	$ 0	1,114
Ending balance (in shares) at Dec. 31, 2016	65,982	65,982
Ending balance at Dec. 31, 2016	$ 133,903	$ 7	106,669	27,227
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of accounting standard (Note 2)	0		409	(409)
Net income	51,614			51,614
Dividends declared	(21,393)			(21,393)
Dividend equivalents on unvested performance-based restricted stock awards	0		110	(110)
Dividend equivalents cancelled upon forfeiture of performance-based restricted stock awards	0		(31)	31
Restricted stock vestings (in shares)		319
Restricted stock vestings	0	$ 0	0
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings (in shares)		(122)
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings	(2,676)	$ 0	(2,676)
Stock-based compensation	5,464		5,464
Shares issuable in lieu of dividend payments on performance-based restricted stock awards (in shares)		5
Shares issuable in lieu of dividend payments on performance-based restricted stock awards	0		0
Issuance of common stock under employee stock purchase plan (in shares)		87
Issuance of common stock under employee stock purchase plan	$ 1,330	$ 0	1,330
Ending balance (in shares) at Dec. 31, 2017	66,271	66,271
Ending balance at Dec. 31, 2017	$ 168,242	$ 7	$ 111,275	$ 56,960