Separate Accounts - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Guaranteed Minimum Withdrawal Benefit
   
Net Amount at Risk by Product and Guarantee [Line Items]    
Value of variable annuity separate accounts $ 100.1 $ 86.6
Separate accounts guarantee liability (0.4) (0.2)
Guaranteed Minimum Death Benefit
   
Net Amount at Risk by Product and Guarantee [Line Items]    
Reserve held for the variable annuity $ 0.1 $ 0.2