Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,735.3	$ 1,712.7
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	103.0	114.3
Percent of Total	5.90%	6.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	156.9	160.0
Percent of Total	9.10%	9.30%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	235.3	224.7
Percent of Total	13.60%	13.10%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	154.1	160.8
Percent of Total	8.90%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	435.2	440.9
Percent of Total	25.10%	25.70%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	78.6	79.6
Percent of Total	4.50%	4.70%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	175.7	159.5
Percent of Total	10.10%	9.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	84.0	90.5
Percent of Total	4.80%	5.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 312.5	$ 282.4
Percent of Total	18.00%	16.50%