Summary of Critical and Significant Accounting Policies - Summary of Merger (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Mar. 31, 2014 USD ($) property	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Apr. 01, 2014 USD ($)
Business Acquisition [Line Items]
Cash consideration for BRE merger			$ 0	$ 555,826	$ 0
BRE Properties, Inc. [Member]
Business Acquisition [Line Items]
Number of property units contributed | property	14
Value of properties contributed	$ 1,400,000
In-place leases acquired	19,000
Cash assumed	140
Rental properties and real estate under development	5,605
Real estate held for sale, net	108
Co-investments	224
Acquired in-place lease value	77
Other assets	16
Mortgage notes payable and unsecured debt	(1,747)
Other liabilities	(87)
Redeemable noncontrolling interest	(5)
Total consideration for BRE merger	4,331					$ 4,300,000
Cash consideration for BRE merger	556
Equity consideration for BRE merger	$ 3,775
Increase in fair value of personal property		$ 13,100
Increase in co-investment purchase price allocation		6,000
Decrease in liability purchase price allocation		$ 7,100