CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net cumulative effect of adoption of accounting standard	$ 208		$ 0	$ 208
Beginning balance (in shares) at Dec. 31, 2017	66,271	66,271
Beginning balance at Dec. 31, 2017	$ 168,242	$ 7	111,275	56,960
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	48,422			48,422
Restricted stock vestings (in shares)		260
Dividends declared	(16,138)			(16,138)
Dividend equivalents on unvested performance-based restricted stock awards	0		68	(68)
Dividend equivalents cancelled upon forfeiture of performance-based restricted stock awards	0		(140)	140
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings (in shares)		(86)
Shares surrendered and cancelled upon vesting of restricted stock to satisfy tax withholdings	(1,209)		(1,209)
Stock-based compensation	4,965		4,965
Issuance of common stock under employee stock purchase plan (in shares)		122
Issuance of common stock under employee stock purchase plan	$ 1,382		1,382
Ending balance (in shares) at Sep. 30, 2018	66,567	66,567
Ending balance at Sep. 30, 2018	$ 205,872	$ 7	$ 116,341	$ 89,524