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Other Assets, Net - Components of Other Assets (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Goodwill And Intangible Assets Disclosure [Abstract]          
Amortization expense $ 8.6 $ 4.9 $ 22.4 $ 11.7  
Straight-line rents receivable, net $ 61.4   $ 61.4   $ 58.2