Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Less: Undistributed earnings attributable to unvested restricted shares	$ (117)	$ (72)	$ (10)	$ (95)
Numerator:
Net income attributable to common shareholders	58,447	55,991	83,744	103,841
Less: Dividends paid on unvested restricted shares	(274)	(238)	(687)	(518)
Less: Undistributed earnings attributable to unvested restricted shares	(117)	(72)	(10)	(95)
Net income attributable to common shareholders excluding amounts attributable to unvested restricted shares	$ 58,056	$ 55,681	$ 83,047	$ 103,228
Denominator:
Weighted-average number of common shares - basic (in shares)	123,544,034	130,670,629	123,641,928	130,969,957
Unvested restricted shares (in shares)	373,039	188,138	387,755	232,682
Unvested performance units (in shares)	25,773	759,926	22,273	745,293
Weighted-average number of common shares - diluted (in shares)	123,942,846	131,618,693	124,051,956	131,947,932
Net income per share attributable to common shareholders - basic (in dollars per share)	$ 0.47	$ 0.43	$ 0.67	$ 0.79
Net income per share attributable to common shareholders - diluted (in dollars per share)	$ 0.47	$ 0.42	$ 0.67	$ 0.78