Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income (loss) attributable to common shareholders	$ 26,420	$ (43,897)	$ 75,919	$ 32,618
Less: dividends paid on unvested share-based compensation	(104)	(121)	(311)	(362)
Undistributed earnings attributable to share-based compensation	0	0	0	0
Net income (loss) available to common shareholders	$ 26,316	$ (44,018)	$ 75,608	$ 32,256
Denominator:
Weighted-average number of common shares-basic (shares)	68,814,805	71,922,904	69,854,618	71,894,313
Effect of dilutive share-based compensation (shares)	388,115	0	373,456	482,036
Weighted-average number of common shares-diluted (shares)	69,202,920	71,922,904	70,228,074	72,376,349
Net income (loss) per share available to common shareholders, basic (usd per share)	$ 0.38	$ (0.61)	$ 1.08	$ 0.45
Net income (loss) per share available to common shareholders, diluted (in usd per share)	$ 0.38	$ (0.61)	$ 1.08	$ 0.45
Antidilutive securities excluded from computation of earnings per share, amount (shares)	6,319	31,311	18,380	117,201