Equity - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator - Basic:
Net income	$ 30,184	$ 31,359	$ 48,242	$ 50,504
Private perpetual preferred unit distributions	(234)	(234)	(468)	(468)
Net income attributable to non-controlling interests	(13,299)	(14,541)	(21,355)	(23,467)
Earnings allocated to unvested shares	(10)	(10)	(16)	(17)
Net income attributable to common stockholders - basic	16,641	16,574	26,403	26,552
Numerator - Diluted:
Net income	30,184	31,359	48,242	50,504
Private perpetual preferred unit distributions	(234)	(234)	(468)	(468)
Earnings allocated to unvested shares	(10)	(10)	(16)	(17)
Net income attributable to common stockholders - diluted	$ 29,940	$ 31,115	$ 47,758	$ 50,019
Denominator:
Weighted average shares outstanding - basic (in shares)	165,256,000	157,921,000	164,311,000	157,265,000
Operating partnership units (in shares)	131,988,000	138,467,000	132,717,000	139,123,000
Effect of dilutive securities:
Stock-based compensation plans (in shares)	0	973,000	1,000	874,000
Exchangeable senior notes (in shares)	0	1,037,000	0	918,000
Weighted average shares outstanding - diluted (in shares)	297,244,000	298,398,000	297,029,000	298,180,000
Earnings per share:
Basic (in USD per share)	$ 0.10	$ 0.10	$ 0.16	$ 0.17
Diluted (in USD per share)	$ 0.10	$ 0.10	$ 0.16	$ 0.17
Antidilutive securities (in shares)	414,801	909,591	417,074	766,078