Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 343,841		$ 343,841		$ 355,735
Other liabilities	33,497		33,497		33,132
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(56,278)	$ (56,812)	(111,139)	$ (111,395)
General and administrative	(11,125)	(10,337)	(25,938)	(20,938)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	4,032,421		4,032,421		3,722,778
Other assets	166,521		166,521		110,333
Total assets	4,198,942		4,198,942		3,833,111
Debt	1,948,724		1,948,724		1,705,051
Other liabilities	64,982		64,982		45,515
Equity	2,185,236		2,185,236		2,082,545
Total liabilities and equity	4,198,942		4,198,942		3,833,111
Company's share of equity	1,291,320		1,291,320		$ 1,155,984
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	81,348	72,111	162,190	148,016
Property operating expenses	(26,567)	(25,866)	(53,636)	(51,275)
Net operating income	54,781	46,245	108,554	96,741
Interest expense	(13,639)	(14,024)	(30,374)	(25,945)
General and administrative	(2,388)	(1,917)	(3,880)	(3,695)
Depreciation and amortization	(31,317)	(28,423)	(62,479)	(56,327)
Net income	7,437	1,881	11,821	10,774
Company's share of net income	15,049	10,308	47,823	21,207
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 300	$ 500	$ 700	$ 1,000