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Derivative Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 6 Months Ended 1 Months Ended 1 Months Ended 1 Months Ended 1 Months Ended
Dec. 31, 2010
Y
Jun. 30, 2013
Dec. 31, 2010
Interest Rate Exchange Agreement Expiring 2010
Apr. 30, 2010
Interest Rate Exchange Agreement Expiring 2010
Apr. 15, 2010
Interest Rate Exchange Agreement Expiring 2010
Oct. 31, 2011
Interest Rate Exchange Agreement Expiring 2011
Jul. 31, 2011
Interest Rate Exchange Agreement Expiring 2011
Jul. 12, 2011
Interest Rate Exchange Agreement Expiring 2011
Dec. 31, 2011
Interest Rate Exchange Agreement Expiring 2012
Sep. 30, 2011
Interest Rate Exchange Agreement Expiring 2012
Sep. 29, 2011
Interest Rate Exchange Agreement Expiring 2012
Dec. 31, 2011
Interest Rate Exchange Agreements Expiring 2011 And 2012
Dec. 06, 2010
4.5 % Senior Notes
Apr. 15, 2010
4.5 % Senior Notes
Derivative [Line Items]                            
Notional amount of derivative contract         $ 300.0     $ 350.0     $ 350.0      
Interest rate exchange agreement expiration date       Dec. 31, 2010     Sep. 30, 2011     Feb. 28, 2012        
Interest rate on senior notes                         4.50% 4.50%
Interest rate contract settlement amount     31.0     34.7     4.0     38.7    
Interest rate contract settlement amount amortization period (in years) 10                          
Interest rate     5.80%                 5.30%    
Interest rate settlement recognized as other expense                       0.8    
Interest rate contract settlement amount to be amortized                       37.9    
Amount to be recognized from hedged transactions within 12 months, approximate   $ 7.3