Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 403,644		$ 403,644		$ 355,735
Other liabilities	33,616		33,616		33,132
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(55,196)	$ (55,938)	(166,335)	$ (167,333)
General and administrative	(10,601)	(9,788)	(36,539)	(30,726)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	4,221,973		4,221,973		3,722,778
Other assets	146,930		146,930		110,333
Total assets	4,368,903		4,368,903		3,833,111
Debt	2,089,414		2,089,414		1,705,051
Other liabilities	101,514		101,514		45,515
Equity	2,177,975		2,177,975		2,082,545
Total liabilities and equity	4,368,903		4,368,903		3,833,111
Company's share of equity	1,267,593		1,267,593		$ 1,155,984
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	84,820	76,303	247,010	224,319
Property operating expenses	(27,779)	(27,753)	(81,415)	(79,028)
Net operating income	57,041	48,550	165,595	145,291
Gain on sale of real estate	0	10,058	0	10,058
Interest expense	(15,805)	(13,718)	(46,179)	(39,663)
General and administrative	(1,254)	(2,452)	(5,134)	(6,147)
Depreciation and amortization	(32,833)	(27,884)	(95,312)	(84,211)
Net income	7,149	14,554	18,970	25,328
Company's share of net income	16,788	19,727	64,611	40,934
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 700	$ 500	$ 1,400	$ 1,500