Common Stock (Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock Disclosures [Abstract]
Basic number of common shares outstanding									40,605,597	40,521,364	40,414,556
Dilutive effect of unvested performance awards									75,624	121,276	120,635
Diluted number of common shares outstanding									40,681,221	40,642,640	40,535,191
Net income	$ 12,942	$ 77,880	$ 26,126	$ 6,089	$ (15,285)	$ 73,271	$ 33,295	$ (6,966)	$ 123,037	$ 84,315	$ 98,261
Income allocated to participating restricted stock									(402)	(297)	(368)
Net income available to common shareholders basic									122,635	84,018	97,893
Income reallocated to participating restricted stock									(402)	(296)	(368)
Net income available to common shareholders diluted									$ 122,635	$ 84,019	$ 97,893
Earnings Per Share, Basic, Distributed									$ 1.515	$ 1.415	$ 1.315
Earnings Per Share, Basic, Undistributed									1.505	0.655	1.105
Basic net income per common share	$ 0.32	$ 1.91	$ 0.64	$ 0.15	$ (0.38)	$ 1.80	$ 0.82	$ (0.17)	3.02	2.070	2.420
Earnings Per Share, Diluted, Distributed									1.515	1.415	1.315
Earnings Per Share, Diluted, Undistributed									1.495	0.655	1.105
Diluted net income per common share	$ 0.32	$ 1.91	$ 0.64	$ 0.15	$ (0.38)	$ 1.79	$ 0.82	$ (0.17)	$ 3.01	$ 2.070	$ 2.420