Income per Share (Summary of Basic and Diluted Earnings per 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]
Net income available to common stockholders	$ 27,951	$ 117,055	$ 67,076	$ 218,783
Weighted average shares/units outstanding-basic (shares)	146,824,268	135,810,060	146,694,916	135,757,584
Potentially dilutive common units:
Stock options (shares)	11,588	30,601	10,887	31,402
Unvested incentive units (shares)	77,946	68,371	70,664	55,742
Forward equity offering (shares)	347,277	0	0	0
Market performance-based awards (shares)	547,189	589,972	640,467	416,267
Weighted average shares/units outstanding-diluted (shares/units)	147,808,268	136,499,004	147,416,934	136,260,995
Income per share:
Basic ($ per share/unit)	$ 0.19	$ 0.86	$ 0.46	$ 1.61
Diluted ($ per share/unit)	$ 0.19	$ 0.86	$ 0.46	$ 1.61