Earnings Per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) (Forward equity [Member]) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
May 2013, Contract [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Forward equity agreements contract date	May 2013
Forward equity agreements settlement date	September and October 2013
Forward Equity Agreement Entered, not included in computation of diluted EPS prior to the date of settlement	39.1
January 2012, Contract [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Forward equity agreements contract date	January 2012
Forward equity agreements settlement date	June 2012
Forward Equity Agreement Entered, not included in computation of diluted EPS prior to the date of settlement	19.0