Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income (loss) Attributable to common shareholders	$ 25,769	$ 26,420	$ 100,234	$ 75,919
Less: dividends paid on unvested share-based compensation	(83)	(104)	(249)	(311)
Undistributed earnings attributable to share based compensation	0	0	(67)	0
Net income (loss) available to common shareholders	$ 25,686	$ 26,316	$ 99,918	$ 75,608
Denominator:
Weighted-average number of common shares-basic (shares)	68,912,185	68,814,805	68,900,402	69,854,618
Effect of dilutive share-based compensation (shares)	343,673	388,115	366,696	373,456
Weighted-average number of common shares-diluted (shares)	69,255,858	69,202,920	69,267,098	70,228,074
Net income (loss) per share available to common shareholders, basic (usd per share)	$ 0.37	$ 0.38	$ 1.45	$ 1.08
Net income (loss) per share available to common shareholders, diluted (in usd per share)	$ 0.37	$ 0.38	$ 1.44	$ 1.08
Antidilutive securities excluded from computation of earnings per share, amount (shares)	0	6,319	4,212	18,380