Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 157,659		$ 157,659		$ 242,326
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Other liabilities	34,945		34,945		34,518
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(55,568)	$ (50,802)	(108,034)	$ (98,348)
General and administrative	(9,698)	(9,549)	(18,880)	(20,094)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,327,936		3,327,936		3,360,360
Other assets	114,524		114,524		96,785
Total assets	3,442,460		3,442,460		3,457,145
Equity Method Investment, Summarized Financial Information, Liabilities and Equity [Abstract]
Debt	1,473,705		1,473,705		1,499,601
Other liabilities	96,151		96,151		92,241
Equity	1,872,604		1,872,604		1,865,303
Total liabilities and equity	3,442,460		3,442,460		3,457,145
Company's share of equity	1,103,272		1,103,272		$ 1,036,047
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	69,180	62,092	144,310	125,589
Property operating expenses	(24,580)	(23,184)	(50,401)	(46,138)
Net operating income	44,600	38,908	93,909	79,451
Gain on sale of real estate	10,796	0	28,291	14
Interest expense	(11,142)	(11,097)	(24,282)	(22,413)
General and administrative	(1,540)	(1,473)	(2,780)	(3,079)
Depreciation and amortization	(25,391)	(24,265)	(54,107)	(49,646)
Net income	17,323	2,073	41,031	4,327
Company's share of net income	14,296	4,472	29,364	8,783
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 900	$ 700	$ 1,700	$ 1,600