Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 542,207		$ 542,207		$ 454,629
Other liabilities	35,374		35,374		33,375
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(54,112)	$ (56,278)	(107,755)	$ (111,139)
General and administrative	(13,927)	(11,125)	(27,386)	(25,938)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	4,419,528		4,419,528		4,367,987
Other assets	81,172		81,172		104,119
Total assets	4,500,700		4,500,700		4,472,106
Debt	2,170,899		2,170,899		2,190,764
Other liabilities	122,353		122,353		106,316
Equity	2,207,448		2,207,448		2,175,026
Total liabilities and equity	4,500,700		4,500,700		4,472,106
Company's share of equity	1,324,081		1,324,081		$ 1,300,140
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	81,832	81,348	165,557	162,190
Property operating expenses	(27,835)	(26,567)	(56,554)	(53,636)
Net operating income	53,997	54,781	109,003	108,554
Interest expense	(16,080)	(13,639)	(31,195)	(30,374)
General and administrative	(2,243)	(2,388)	(4,171)	(3,880)
Depreciation and amortization	(28,937)	(31,317)	(58,872)	(62,479)
Net income	6,737	7,437	14,765	11,821
Company's share of net income	16,959	15,049	33,235	47,823
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 1,800	$ 300	$ 3,500	$ 700