Equity - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 39,781	$ 29,230	$ 30,184	$ 18,058	$ 32,260	$ 35,489	$ 31,359	$ 19,145	$ 33,008	$ 32,897	$ 24,640	$ 16,705	$ 117,253	$ 118,253	$ 107,250
Private perpetual preferred unit distributions													(936)	(936)	(936)
Net income attributable to non-controlling interests													(50,714)	(54,670)	(54,858)
Earnings allocated to unvested shares													(38)	(36)	(36)
Net income attributable to common stockholders - basic													65,565	62,611	51,420
Earnings allocated to unvested shares													(38)	(36)	(36)
Net income attributable to common stockholders - diluted													$ 116,279	$ 117,281	$ 106,278
Denominator:
Weighted average shares outstanding - basic (shares)													167,571,000	158,380,000	133,881,000
Operating partnership units (shares)													129,687,000	138,075,000	142,967,000
Effect of dilutive securities, stock-based compensation plans (shares)													1,000	775,000	454,000
Effect of dilutive securities, exchangeable senior notes (shares)													0	819,000	266,000
Weighted average shares outstanding - dilutive (in shares)													297,259,000	298,049,000	277,568,000
Basic (USD per share)													$ 0.39	$ 0.40	$ 0.38
Diluted (USD per share)													$ 0.39	$ 0.39	$ 0.38
Antidilutive securities (shares)													485,865	834,267	800,746