Income per Share (Summary of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income available to common stockholders	$ 77,682	$ 187,330	$ 27,951	$ 39,125	$ (40,039)	$ 38,522	$ 117,055	$ 101,728	$ 332,088	$ 217,266	$ 132,718
Weighted average units outstanding—basic (in shares)									149,953,662	138,247,606	133,369,047
Potentially dilutive common shares:
Stock options (in shares)									9,726	20,424	30,434
Unvested incentive units (in shares)									71,031	95,746	90,449
Forward equity offering (in shares)									3,990	0	0
Market performance-based awards (in shares)									641,279	501,645	147,305
Weighted average units outstanding—diluted (in shares)									150,679,688	138,865,421	133,637,235
Income per share:
Basic (in dollars per share)	$ 0.49	$ 1.27	$ 0.19	$ 0.27	$ (0.28)	$ 0.28	$ 0.86	$ 0.75	$ 2.21	$ 1.57	$ 1.00
Diluted (in dollars per share)	$ 0.49	$ 1.25	$ 0.19	$ 0.27	$ (0.28)	$ 0.28	$ 0.86	$ 0.75	$ 2.20	$ 1.56	$ 0.99