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Earnings Per Common Share Earnings Per Common 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
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net earnings (loss)	$ 34,824	$ (12,599)	$ (8,893)	$ (6,679)	$ (146,383)	$ 6,181	$ 9,946	$ 25,061	$ 6,653	$ (105,195)	$ (953,520)
Weighted-average shares outstanding									53,282	53,121	53,050
Effect of dilutive stock awards	562									0	0
Weighted-average dilutive shares									53,844	53,121	53,050
Basic earnings (loss) per common share	$ 0.65	$ (0.24)	$ (0.17)	$ (0.13)	$ (2.76)	$ 0.12	$ 0.19	$ 0.47	$ 0.12	$ (1.98)	$ (17.97)
Diluted earnings (loss) per common share	$ 0.65	$ (0.24)	$ (0.17)	$ (0.13)	$ (2.76)	$ 0.12	$ 0.19	$ 0.47	$ 0.12	$ (1.98)	$ (17.97)
Performance share units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings (loss per common share									329	880	611
Restricted share units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings (loss per common share	0									482	257
Stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted earnings (loss per common share									2,554	3,072	2,586