Subsequent Events - Additional Information (Detail) (USD $)	3 Months Ended			9 Months Ended					3 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Oct. 18, 2013	Oct. 02, 2013	Dec. 31, 2012	Dec. 31, 2013 Noncontrolling Interest [Member]	Nov. 01, 2013 Greenwood Forest [Member] Apartments	Nov. 01, 2013 Whispering Pines [Member] Apartments	Oct. 16, 2013 Unsecured Debt [Member] Senior Notes [Member]	Sep. 30, 2013 Unsecured Debt [Member] Senior Notes [Member]	Sep. 30, 2013 Unsecured Debt [Member] Senior Notes [Member] Tranche 1 [Member]	Sep. 30, 2013 Unsecured Debt [Member] Variable Rate Debt
Subsequent Event [Line Items]
Business Acquisition, Cost of Acquired Entity, Purchase Price							$ 2,200,000,000
Stock Issued During Period, Shares, Acquisitions	31,916,765								2,574,631
Share Price							$ 62.56
Merger Related Expenses		5,561,000	0	11,298,000	0
Notes Payable		1,860,000,000		1,860,000,000				1,670,000,000							350,000,000
Number of Units in Real Estate Property		48,343		48,343						251	216
Payments for (Proceeds from) Derivative Instrument, Investing Activities	9,600,000
Debt Instrument, Face Amount												350,000,000	175,000,000	18,000,000
Debt Instrument, Interest Rate, Stated Percentage												4.30%		3.15%
Debt Instrument, Issue Price												0.99047
Preferred Stock, Shares Issued						125
Preferred Units, Dividend Rate, Percentage	12.50%
Preferred Stock, Value, Issued						$ 1,000