Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted net loss per common share calculation:
Net income (loss)									$ 63,087	$ 114,205	$ (193,420)
Weighted average common shares outstanding - basic (in shares)									37,061	35,919	35,486
Plus: net effect of dilutive stock options and unvested restricted stock units (in shares)									27	88	0
Weighted average common shares outstanding - dilutive (in shares)									37,088	36,007	35,486
Net income (loss) per common share - basic (USD per share)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (0.26)	$ 5.26	$ (0.62)	$ (1.20)	$ 1.70	$ 3.18	$ (5.45)
Net income (loss) per common share - diluted (USD per share)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (0.26)	$ 5.25	$ (0.62)	$ (1.20)	$ 1.70	$ 3.17	$ (5.45)
Anti-dilutive securities
Total (in shares)									6,474	5,361	4,080
Stock options outstanding
Anti-dilutive securities
Total (in shares)									5,873	5,179	3,359
Restricted stock units
Anti-dilutive securities
Total (in shares)									601	182	721