Earnings Per Share (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
Numerator:
Net income (loss) Attributable to common shareholders	$ 7,875	$ 26,420	$ 39,488	$ 10,011	$ 14,334	$ (43,897)	$ 69,949	$ 6,566	$ 83,794	$ 46,952	$ 68,718
Less: dividends paid on unvested share-based compensation									(415)	(483)	(432)
Net income (loss) available to common shareholders									$ 83,379	$ 46,469	$ 68,286
Denominator:
Weighted-average number of common shares-basic (shares)									69,591,973	71,901,499	71,715,870
Effect of dilutive share-based compensation (shares)									392,864	471,743	668,419
Weighted-average number of common shares-diluted (shares)									69,984,837	72,373,242	72,384,289
Net income (loss) per share available to common shareholders, basic (usd per share)	$ 0.11	$ 0.38	$ 0.57	$ 0.14	$ 0.20	$ (0.61)	$ 0.97	$ 0.09	$ 1.20	$ 0.65	$ 0.95
Net income (loss) per share available to common shareholders, diluted (in usd per share)	$ 0.11	$ 0.38	$ 0.57	$ 0.14	$ 0.20	$ (0.61)	$ 0.96	$ 0.09	$ 1.19	$ 0.64	$ 0.94
Antidilutive securities excluded from computation of earnings per share, amount (shares)									6,319	114,889	0