Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 239,685		$ 190,505
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Other liabilities	34,027		32,922
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(54,583)	$ (52,466)
General and administrative	(10,601)	(9,182)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,433,014		3,807,245
Other assets	102,385		121,505
Total assets	3,535,399		3,928,750
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt	1,461,893		1,617,639
Other liabilities	75,570		74,607
Equity	1,997,936		2,236,504
Total liabilities and equity	3,535,399		3,928,750
Company's share of equity	1,035,724		$ 1,161,275
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	75,905	75,130
Property operating expenses	(25,408)	(25,821)
Net operating income	50,497	49,309
Gain on sale of real estate	0	17,495
Interest expense	(11,921)	(13,140)
General and administrative	(1,778)	(1,240)
Depreciation and amortization	(27,904)	(28,716)
Net income	8,894	23,708
Company's share of net income	10,899	15,068
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 500	$ 800