Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 170,972		$ 170,972		$ 242,326
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Other liabilities	34,172		34,172		34,518
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(56,693)	$ (50,053)	(164,727)	$ (148,401)
General and administrative	(9,647)	(11,129)	(28,527)	(31,223)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,432,866		3,432,866		3,360,360
Other assets	118,131		118,131		96,785
Total assets	3,550,997		3,550,997		3,457,145
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt	1,506,107		1,506,107		1,499,601
Other liabilities	90,039		90,039		92,241
Equity	1,954,851		1,954,851		1,865,303
Total liabilities and equity	3,550,997		3,550,997		3,457,145
Company's share of equity	1,122,913		1,122,913		$ 1,036,047
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	72,124	65,869	216,434	191,458
Property operating expenses	(24,976)	(23,094)	(75,377)	(69,232)
Net operating income	47,148	42,775	141,057	122,226
Gain on sale of real estate	0	0	28,291	14
Interest expense	(10,978)	(11,314)	(35,260)	(33,727)
General and administrative	(1,496)	(1,335)	(4,276)	(4,414)
Depreciation and amortization	(25,569)	(26,574)	(79,676)	(76,220)
Net income	9,105	3,552	50,136	7,879
Company's share of net income	9,568	7,179	38,932	15,962
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 800	$ 800	$ 2,500	$ 2,900