Significant Transactions - Asset Management Business Disposition (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012 Property
Napico Portfolio [Member]
Significant Acquisitions and Disposals [Line Items]
Number of partnerships in legally sold portfolio	17
Non-recourse debt payable to a noncontrolling limited partner was contributed to the partnership's capital	$ 9.2
Derecognition Of Remaining Noncontrolling Interest Balances Resulting In Adjustment To Net Income Attributable To Parent		$ 20.6
Unconsolidated Properties [Member]
Significant Acquisitions and Disposals [Line Items]
Number of apartment communities	11	20	22
Unconsolidated Properties [Member] | Napico Portfolio [Member]
Significant Acquisitions and Disposals [Line Items]
Number of apartment communities	57
Wholly Owned And Consolidated Properties [Member] | Napico Portfolio [Member]
Significant Acquisitions and Disposals [Line Items]
Number of apartment communities	14