Earnings Per Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 1,780	$ 25,576	$ 14,097	$ 15,845	$ 11,649	$ (9,885)	$ 19,917	$ 10,089	$ 57,298	$ 31,770	$ 12,678
Weighted average common shares outstanding - Basic									38,655	38,207	38,224
Dilutive effect of stock-based awards									732	436	282
Weighted average common shares outstanding - Diluted									39,387	38,643	38,506
Earnings per common share - Basic	$ 0.05	$ 0.66	$ 0.36	$ 0.41	$ 0.30	$ (0.26)	$ 0.52	$ 0.27	$ 1.48	$ 0.83	$ 0.33
Earnings per common share - Diluted	$ 0.05	$ 0.65	$ 0.36	$ 0.40	$ 0.30	$ (0.26)	$ 0.52	$ 0.26	$ 1.45	$ 0.82	$ 0.33