Earnings 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	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per common share
Net income									$ 39,009	$ 44,176	$ 39,961
Less income allocated to participating securities									540	604	609
Net income attributable to common shareholders	$ 10,101	$ 7,302	$ 16,328	$ 5,278	$ 15,141	$ 15,548	$ 13,104	$ 383	$ 38,469	$ 43,572	$ 39,352
Weighted average common shares outstanding									22,480,679	22,329,044	22,168,500
Basic income per common share (in dollars per share)	$ 0.44	$ 0.32	$ 0.72	$ 0.23	$ 0.67	$ 0.69	$ 0.58	$ 0.02	$ 1.71	$ 1.95	$ 1.78
Earnings per common share assuming dilution
Net income									$ 39,009	$ 44,176	$ 39,961
Less income allocated to participating securities									540	604	609
Net income attributable to common shareholders	$ 10,101	$ 7,302	$ 16,328	$ 5,278	$ 15,141	$ 15,548	$ 13,104	$ 383	$ 38,469	$ 43,572	$ 39,352
Weighted average common shares outstanding									22,480,679	22,329,044	22,168,500
Incremental shares applicable to stock based compensation										12,731	20,346
Weighted average common shares assuming dilution									22,480,679	22,341,775	22,188,846
Earnings per common share assuming dilution (in dollars per share)	$ 0.44	$ 0.32	$ 0.71	$ 0.23	$ 0.66	$ 0.68	$ 0.57	$ 0.01	$ 1.70	$ 1.94	$ 1.77