Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 263,284		$ 263,284		$ 190,505
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Other liabilities	33,988		33,988		32,922
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(56,812)	$ (55,568)	(111,395)	$ (108,034)
General and administrative	(10,337)	(9,698)	(20,938)	(18,880)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,460,610		3,460,610		3,807,245
Other assets	86,369		86,369		121,505
Total assets	3,546,979		3,546,979		3,928,750
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt	1,416,292		1,416,292		1,617,639
Other liabilities	69,128		69,128		74,607
Equity	2,061,559		2,061,559		2,236,504
Total liabilities and equity	3,546,979		3,546,979		3,928,750
Company's share of equity	1,045,059		1,045,059		$ 1,161,275
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	72,111	69,180	148,016	144,310
Property operating expenses	(25,866)	(24,580)	(51,275)	(50,401)
Net operating income	46,245	44,600	96,741	93,909
Gain on sale of real estate	0	10,796	0	28,291
Interest expense	(14,024)	(11,142)	(25,945)	(24,282)
General and administrative	(1,917)	(1,540)	(3,695)	(2,780)
Depreciation and amortization	(28,423)	(25,391)	(56,327)	(54,107)
Net income	1,881	17,323	10,774	41,031
Company's share of net income	10,308	14,296	21,207	29,364
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 500	$ 900	$ 1,000	$ 1,700