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Borrowing Limits, Amounts Outstanding and Amounts Available Under Revolving Credit Facility (Detail) (Revolving Facility, USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Revolving Facility
     
Debt Instrument [Line Items]      
Revolving credit facility $ 60.0 $ 60.0 $ 115.0
Outstanding revolving credit facility 32.6 49.4 77.6
Available revolving credit facility $ 27.4 $ 10.6 $ 37.4