Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,776.1	$ 1,712.7
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	102.0	114.3
Percent of Total	5.70%	6.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	166.8	160.0
Percent of Total	9.40%	9.30%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	241.0	224.7
Percent of Total	13.60%	13.10%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	168.4	160.8
Percent of Total	9.50%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	433.9	440.9
Percent of Total	24.40%	25.70%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	68.2	79.6
Percent of Total	3.80%	4.70%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	202.4	159.5
Percent of Total	11.40%	9.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	83.2	90.5
Percent of Total	4.70%	5.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 310.2	$ 282.4
Percent of Total	17.50%	16.50%