Income (Loss) Per Share - Commutation of Basic and Diluted Loss per 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
Earnings Per Share [Abstract]
Net income (loss)	$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 19,005	$ 18,824	$ 7,006	$ 27,786	$ 39,427	$ 72,621	$ (45,835)
Undistributed income allocated to participating securities									556	878	0
Net income (loss) attributable to common stockholders									38,871	71,743	(45,835)
Plus: Net income effect of convertible securities									0	0	0
Numerator for diluted income (loss) per common share									$ 38,871	$ 71,743	$ (45,835)
Basic weighted-average common stock shares outstanding (in shares)									45,726	45,543	42,349
Add dilutive effects of common stock equivalents (in shares)									29	40	0
Diluted weighted-average common stock shares outstanding (in shares)									45,755	45,583	42,349
Basic (USD per share)	$ 0.30	$ (0.13)	$ 0.35	$ 0.33	$ 0.41	$ 0.41	$ 0.15	$ 0.60	$ 0.85	$ 1.58	$ (1.08)
Diluted (USD per share)	$ 0.30	$ (0.13)	$ 0.35	$ 0.33	$ 0.41	$ 0.41	$ 0.15	$ 0.58	$ 0.85	$ 1.57	$ (1.08)