Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 395,710		$ 355,735
Other liabilities	33,477		33,132
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(54,861)	$ (54,583)
General and administrative	(14,813)	(10,601)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	3,794,212		3,722,778
Other assets	161,538		110,333
Total assets	3,955,750		3,833,111
Debt	1,744,663		1,705,051
Other liabilities	69,782		45,515
Equity	2,141,305		2,082,545
Total liabilities and equity	3,955,750		3,833,111
Company's share of equity	1,290,957		$ 1,155,984
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	80,842	75,905
Property operating expenses	(27,069)	(25,408)
Net operating income	53,773	50,497
Interest expense	(16,735)	(11,921)
General and administrative	(1,492)	(1,778)
Depreciation and amortization	(31,162)	(27,904)
Net income	4,384	8,894
Company's share of net income	32,774	10,899
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 400	$ 500