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Derivative Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Put Option [Member]
Jun. 30, 2013
Interest rate swaps [Member]
interest_rate_swap_agreements
Jun. 30, 2012
Interest rate swaps [Member]
Jun. 30, 2013
Interest rate swaps [Member]
interest_rate_swap_agreements
Jun. 30, 2012
Interest rate swaps [Member]
Feb. 13, 2013
Derivatives designated as hedging instruments [Member]
Interest rate caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]              
Number of interest rate swap agreements     8   8    
Notional amount of interest rate swap agreements     $ 5,750.0   $ 5,750.0   $ 4,664.1
LIBOR cap rate             4.50%
Aggregate cash payment   13.9          
Increase (decrease) in interest expense     42.8 42.2 85.0 84.0  
Variable rate debt hedged using interest rate swap agreements $ 5,750.0            
Variable rate debt, percentage of total debt 18.00%            
Fixed-rate debt, percentage of total debt 82.00%