Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 145,711		$ 242,326
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Other liabilities	34,814		34,518
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(52,466)	$ (47,546)
General and administrative	(9,182)	(10,545)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,323,201		3,360,360
Other assets	111,040		96,785
Total assets	3,434,241		3,457,145
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt	1,475,295		1,499,601
Other liabilities	97,765		92,241
Equity	1,861,181		1,865,303
Total liabilities and equity	3,434,241		3,457,145
Company's share of equity	1,069,684		$ 1,036,047
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	75,130	63,498
Property operating expenses	(25,821)	(22,954)
Net operating income	49,309	40,544
Gain on sale of real estate	17,495	14
Interest expense	(13,140)	(11,316)
General and administrative	(1,240)	(1,606)
Depreciation and amortization	(28,716)	(25,381)
Net income	23,708	2,255
Company's share of net income	15,068	4,311
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 800	$ 900