Subsequent Events (Details)	1 Months Ended			12 Months Ended
	Feb. 26, 2016 USD ($) unit	Jan. 31, 2016 USD ($) unit	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) unit	Dec. 31, 2014 USD ($) unit	Dec. 31, 2013 USD ($) unit
Subsequent Event [Line Items]
Purchase of parcel of land				$ 515,726,000	$ 387,547,000	$ 348,774,000
Number of units sold | unit				848	594	363
Sale of apartment community			$ 13,000,000	$ 308,800,000	$ 120,400,000	$ 57,500,000
Debt instrument, face amount				$ 2,400,000,000	$ 1,900,000,000
Subsequent Event [Member] | Unsecured Debt [Member] | Unsecured Bonds 4.36% [Member]
Subsequent Event [Line Items]
Debt instrument, face amount		$ 150,000,000.0
Interest rate, stated percentage		4.36%
Subsequent Event [Member] | Palo Alto, CA [Member]
Subsequent Event [Line Items]
Proceeds from sale of headquarters building		$ 18,000,000
Subsequent Event [Member] | Mio Apartment Community [Member]
Subsequent Event [Line Items]
Number of units acquired | unit		103
Purchase of parcel of land		$ 51,300,000
Subsequent Event [Member] | Harvest Park [Member]
Subsequent Event [Line Items]
Purchase of parcel of land	$ 30,500,000
Number of units sold | unit	104
Subsequent Event [Member] | BEXAWE, LLC [Member] | The Heights [Member]
Subsequent Event [Line Items]
Number of units sold | unit		332
Sale of apartment community		$ 93,800,000
Repayments of loan on property		$ 50,300,000
Equity method investment, ownership percentage		50.00%