Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,730.4	$ 1,712.7
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	113.2	114.3
Percent of Total	6.50%	6.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	158.9	160.0
Percent of Total	9.20%	9.30%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	233.1	224.7
Percent of Total	13.50%	13.10%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	158.0	160.8
Percent of Total	9.10%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	437.8	440.9
Percent of Total	25.30%	25.70%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	79.1	79.6
Percent of Total	4.60%	4.70%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	157.8	159.5
Percent of Total	9.10%	9.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	89.6	90.5
Percent of Total	5.20%	5.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 302.9	$ 282.4
Percent of Total	17.50%	16.50%