Earnings per Share - Reconciliation (Details) - USD ($) $ / shares in Units, $ 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]
Numerator for basic and diluted EPS — net income (loss) allocated to common stockholders	$ (26,200)	$ (5,900)	$ 6,300	$ (800)	$ 23,700	$ 13,100	$ 19,700	$ 4,400	$ (26,651)	$ 60,762	$ 25,598
Denominator for basic EPS — weighted average number of shares (shares)									32,074,894	32,042,594	31,197,288
Add — dilutive effect of:
Weighted average number of incremental shares issuable from restricted stock units (shares)									0	31,000	3,000
Denominator for diluted EPS — weighted average number of shares and dilutive potential shares (shares)									32,074,894	32,074,232	31,200,378
Earnings (loss) per share:
Basic (usd per share)	$ (0.85)	$ (0.18)	$ 0.20	$ (0.02)	$ 0.74	$ 0.41	$ 0.62	$ 0.14	$ (0.83)	$ 1.90	$ 0.82
Diluted (usd per share)	$ (0.85)	$ (0.18)	$ 0.20	$ (0.02)	$ 0.74	$ 0.41	$ 0.62	$ 0.14	$ (0.83)	$ 1.89	$ 0.82