Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 313,825		$ 313,825		$ 190,505
Other liabilities	33,733		33,733		32,922
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(55,938)	$ (56,693)	(167,333)	$ (164,727)
General and administrative	(9,788)	(9,647)	(30,726)	(28,527)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,671,101		3,671,101		3,807,245
Other assets	111,372		111,372		121,505
Total assets	3,782,473		3,782,473		3,928,750
Debt	1,597,597		1,597,597		1,617,639
Other liabilities	65,865		65,865		74,607
Equity	2,119,011		2,119,011		2,236,504
Total liabilities and equity	3,782,473		3,782,473		3,928,750
Company's share of equity	1,088,332		1,088,332		$ 1,161,275
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	76,303	72,124	224,319	216,434
Property operating expenses	(27,753)	(24,976)	(79,028)	(75,377)
Net operating income	48,550	47,148	145,291	141,057
Gain on sale of real estate	10,058	0	10,058	28,291
Interest expense	(13,718)	(10,978)	(39,663)	(35,260)
General and administrative	(2,452)	(1,496)	(6,147)	(4,276)
Depreciation and amortization	(27,884)	(25,569)	(84,211)	(79,676)
Net income	14,554	9,105	25,328	50,136
Company's share of net income	19,727	9,568	40,934	38,932
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 500	$ 800	$ 1,500	$ 2,500