Earnings Per Share and Capital Structure Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended		9 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	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income (loss) available to common shareholders	$ 8,986	$ 12,640	$ (14,346)	$ 5,398	$ (47,041)	$ 3,585	$ 20,836	$ (1,050)	$ (8,948)	$ 19,786	$ 3,692	$ 23,371	$ 12,678	$ (23,670)	$ (153,153)
Weighted average common shares outstanding - Basic		38,114	38,434	38,442		39,213	39,356	39,235	38,438	39,296	38,329	39,268	38,224	39,184	39,281
Dilutive effect of stock-based awards													282	0	0
Weighted average common shares outstanding - Diluted		38,358	38,434	38,758		39,493	39,544	39,235	38,438	39,528	38,591	39,516	38,506	39,184	39,281
Earnings (loss) per common share - Basic(1)	$ 0.23	$ 0.33	$ (0.37)	$ 0.14	$ (1.21)	$ 0.09	$ 0.53	$ (0.03)	$ (0.23)	$ 0.50	$ 0.10	$ 0.60	$ 0.33	$ (0.60)	$ (3.90)
Earnings (loss) per common share - Diluted	$ 0.23	$ 0.33	$ (0.37)	$ 0.14	$ (1.21)	$ 0.09	$ 0.53	$ (0.03)	$ (0.23)	$ 0.50	$ 0.10	$ 0.59	$ 0.33	$ (0.60)	$ (3.90)