Mortgage Loans, Sorted by Applicable Credit Quality Indicators, Internal rating (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,815.1	[1]	$ 1,712.7
B Credit Rating [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	13.1		13.1
Ba Credit Rating [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	13.5		34.2
Baa Credit Rating [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	1,094.6		994.5
A Credit Rating [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	683.1		659.4
AA Credit Rating [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 10.8		$ 11.5

[1] (1)The amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and/or accretion of discounts.