Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended							12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income	$ 21,347	$ 24,273	$ 26,829	$ 25,543	$ 24,257	$ 26,897	$ 28,412	$ 97,992	$ 104,923	$ 36,453
Less: Net income allocated to unvested participating restricted stock								(1,387)	(1,400)	(521)
Net income available to common stockholders - basic								96,605	103,523	35,932
Effect of unvested participating restricted stock								2	4	0
Numerator - net income available to common shareholders - diluted								$ 96,607	$ 103,527	$ 35,932
Weighted average outstanding shares of common stock - basic (shares)								18,030	18,637	18,753
Stock options (shares)								95	131	203
Weighted average outstanding shares of common stock - diluted (shares)								18,125	18,768	18,956
Basic (USD per share)	$ 1.18	$ 1.33	$ 1.46	$ 1.38	$ 1.29	$ 1.41	$ 1.49	$ 5.36	$ 5.55	$ 1.92
Diluted (USD per share)	$ 1.18	$ 1.33	$ 1.45	$ 1.37	$ 1.28	$ 1.40	$ 1.47	$ 5.33	$ 5.52	$ 1.90