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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income available to common stockholders	$ 53,306	$ (4,140)	$ 57,837	$ 66,145	$ 77,682	$ 187,330	$ 27,951	$ 39,125	$ 173,148	$ 332,088	$ 217,266
Weighted average units outstanding—basic (in shares)									174,059,386	149,953,662	138,247,606
Potentially dilutive common shares:
Stock options (in shares)									0	9,726	20,424
Unvested incentive units (in shares)									141,136	71,031	95,746
Forward equity offering (in shares)									124,527	3,990	0
Market performance-based awards (in shares)									570,049	641,279	501,645
Weighted average units outstanding—diluted (in shares)									174,895,098	150,679,688	138,865,421
Income per share:
Basic (in dollars per share)	$ 0.26	$ (0.02)	$ 0.36	$ 0.42	$ 0.49	$ 1.27	$ 0.19	$ 0.27	$ 0.99	$ 2.21	$ 1.57
Diluted (in dollars per share)	$ 0.26	$ (0.02)	$ 0.36	$ 0.41	$ 0.49	$ 1.25	$ 0.19	$ 0.27	$ 0.99	$ 2.20	$ 1.56