Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income attributable to common shareholders	$ 42,246	$ 58,447	$ 64,004	$ 83,744
Less: Dividends paid on unvested restricted shares	(273)	(274)	(555)	(687)
Less: Undistributed Earnings allocated to unvested restricted shares	7	117	0	10
Net income attributable to common shareholders excluding amounts attributable to unvested restricted shares	$ 41,966	$ 58,056	$ 63,449	$ 83,047
Denominator:
Weighted-average number of common shares - basic (in shares)	123,785,735	123,544,034	123,760,096	123,641,928
Unvested restricted shares (in shares)	86,027	373,039	96,292	387,755
Incremental Common Shares Attributable to Performance Units	0	25,773	0	22,273
Weighted-average number of common shares - diluted (in shares)	123,871,762	123,942,846	123,856,388	124,051,956
Net income per share attributable to common shareholders - basic (in dollars per share)	$ 0.34	$ 0.47	$ 0.51	$ 0.67
Net income per share attributable to common shareholders - diluted (in dollars per share)	$ 0.34	$ 0.47	$ 0.51	$ 0.67