Co-investments - Combined Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	$ 497,794		$ 454,629
Other liabilities	33,829		33,375
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Interest expense	(53,643)	$ (54,861)
General and administrative	(13,459)	(14,813)
Total co investment [Member]
Equity Method Investment, Summarized Financial Information, Assets [Abstract]
Rental properties and real estate under development	4,310,726		4,367,987
Other assets	101,790		104,119
Total assets	4,412,516		4,472,106
Debt	2,160,347		2,190,764
Other liabilities	114,300		106,316
Equity	2,137,869		2,175,026
Total liabilities and equity	4,412,516		4,472,106
Company's share of equity	1,307,561		$ 1,300,140
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Property revenues	83,725	80,842
Property operating expenses	(28,719)	(27,069)
Net operating income	55,006	53,773
Interest expense	(15,115)	(16,735)
General and administrative	(1,928)	(1,492)
Depreciation and amortization	(29,935)	(31,162)
Net income	8,028	4,384
Company's share of net income	16,276	32,774
Total co investment [Member] | Affiliated Entity [Member]
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Company's share of net income	$ 1,700	$ 400