Real Estate Investments - Preferred Equity Investments and Real Estate Under Development (Details) $ in Thousands	1 Months Ended			12 Months Ended
	Oct. 31, 2017 USD ($)	Aug. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) investment project	Dec. 31, 2016 USD ($) investment	Dec. 31, 2015 USD ($)	Dec. 31, 2017 investment	Dec. 31, 2017 unit	Dec. 31, 2017 project
Schedule of Equity Method Investments [Line Items]
Co-investment				$ 1,155,984	$ 1,161,275
Number of investments with a commitment to fund | investment					5
Equity method investment, funded amount					$ 116,100
Non-operating distributions from co-investments				162,439	76,231	$ 31,938
Gain on disposal of equity method investment				$ 44,837	$ 13,046	$ 0
Full redemption of a preferred equity investment	$ 5,000	$ 11,700
Number of development projects | project				5
Ownership interests, number of active development projects							2		7
Number of units to be developed | unit								1,982
Real Estate predevelopment projects [Member]
Schedule of Equity Method Investments [Line Items]
Actual cost of active development projects				$ 557,000
Minimum [Member]
Schedule of Equity Method Investments [Line Items]
Annualized preferred return rate				9.50%	10.00%
Maximum [Member]
Schedule of Equity Method Investments [Line Items]
Annualized preferred return rate				11.30%	12.00%
Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Commitment to fund preferred equity investment in the project				$ 153,800
Number of investments with a commitment to fund | investment				8
Equity method investment, funded amount				$ 77,500
Total preferred interest investments [Member]
Schedule of Equity Method Investments [Line Items]
Co-investment				265,200	$ 222,100
Preferred Equity Investment In A Joint Venture That Holds A Property In Seattle, Washington [Member]
Schedule of Equity Method Investments [Line Items]
Non-operating distributions from co-investments			$ 12,600
Reduction in preferred equity method			(12,400)
Gain on disposal of equity method investment			$ 300
Preferred Equity Investment In Joint Venture That Holds A Property In San Jose, CA [Member]
Schedule of Equity Method Investments [Line Items]
Partial redemption of equity method investment		$ 6,900
Equity method investments				$ 13,400
Preferred Equity Investment In A Joint Venture That Holds A Property In Concord, CA [Member]
Schedule of Equity Method Investments [Line Items]
Full redemption of a preferred equity investment	5,100
Proceeds from early redemption fee	$ 100