Common Stock (Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock Disclosures [Abstract]
Basic number of common shares outstanding									40,274,986	40,190,991	40,114,594
Dilutive effect of unvested performance awards									33,576	20,726	12,053
Diluted number of common shares outstanding									40,308,562	40,211,717	40,126,647
Net income	$ 648	$ 56,740	$ 21,072	$ 3,458	$ 4,241	$ 52,476	$ 30,096	$ 4,615	$ 81,918	$ 91,428	$ 88,583
Income allocated to participating restricted stock									(243)	(301)	(254)
Net income available to common shareholders basic									81,675	91,127	88,329
Income reallocated to participating restricted stock									(243)	(301)	(254)
Net income available to common shareholders diluted									$ 81,675	$ 91,127	$ 88,329
Earnings Per Share, Basic, Distributed									$ 1.165	$ 1.105	$ 1.045
Earnings Per Share, Basic, Undistributed									0.865	1.165	1.155
Basic net income per common share	$ 0.02	$ 1.40	$ 0.52	$ 0.09	$ 0.10	$ 1.30	$ 0.75	$ 0.11	2.030	2.270	2.200
Earnings Per Share, Diluted, Distributed									1.165	1.105	1.045
Earnings Per Share, Diluted, Undistributed									0.865	1.165	1.155
Diluted net income per common share	$ 0.02	$ 1.40	$ 0.52	$ 0.09	$ 0.10	$ 1.30	$ 0.75	$ 0.11	$ 2.030	$ 2.270	$ 2.200