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Long Term Debt (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Total principal debt $ 906.0 $ 851.9
Unamortized debt discount (2.1) (2.5)
Total 903.9 849.4
Senior debentures maturing June 15, 2021
   
Debt Instrument [Line Items]    
Total principal debt 300.0 300.0
Senior debentures maturing March 1, 2020
   
Debt Instrument [Line Items]    
Total principal debt 165.1 200.0
Senior debentures maturing October 15, 2025
   
Debt Instrument [Line Items]    
Total principal debt 81.2 120.9
Subordinated debentures maturing March 30, 2053
   
Debt Instrument [Line Items]    
Total principal debt 175.0  
Subordinated debentures maturing February 3, 2027
   
Debt Instrument [Line Items]    
Total principal debt 59.7 59.7
Federal Home Loan Bank of Boston
   
Debt Instrument [Line Items]    
Total principal debt $ 125.0 $ 171.3