Earnings per Share - Reconciliation (Details) - USD ($) $ / shares in Units, $ 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]
Numerator for basic and diluted EPS — net income (loss)	$ 11,000	$ 11,200	$ 10,600	$ 6,300	$ (26,200)	$ (5,900)	$ 6,300	$ (800)	$ 39,222	$ (26,651)	$ 60,762
Denominator for basic EPS — weighted average number of shares (shares)									26,813,285	32,074,894	32,042,594
Add — dilutive effect of:
Restricted stock units and Cogent Earnout shares									825,000	0	31,000
Denominator for diluted EPS — weighted average number of shares and dilutive potential shares (shares)									27,637,720	32,074,894	32,074,232
Earnings (loss) per share:
Basic (usd per share)	$ 0.46	$ 0.45	$ 0.39	$ 0.21	$ (0.85)	$ (0.18)	$ 0.20	$ (0.02)	$ 1.46	$ (0.83)	$ 1.90
Diluted (usd per share)	$ 0.45	$ 0.43	$ 0.38	$ 0.21	$ (0.85)	$ (0.18)	$ 0.20	$ (0.02)	$ 1.42	$ (0.83)	$ 1.89