Mortgage Loans, Sorted by Applicable Credit Quality Indicators, Internal rating (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,843.3	$ 1,815.1
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	10.6	10.8
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	666.8	683.1
Baa Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	1,121.3	1,094.6
Ba Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	13.4	13.5
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans	$ 31.2	$ 13.1