Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 1,606.9	$ 1,516.8
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	148.2	146.8
Percent of Total	9.20%	9.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	172.3	184.8
Percent of Total	10.70%	12.20%
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	214.0	171.7
Percent of Total	13.30%	11.30%
West North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	152.4	134.6
Percent of Total	9.50%	8.90%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	390.5	372.0
Percent of Total	24.30%	24.50%
East South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	52.7	26.9
Percent of Total	3.30%	1.80%
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	161.9	171.8
Percent of Total	10.10%	11.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	59.2	60.7
Percent of Total	3.70%	4.00%
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 255.7	$ 247.5
Percent of Total	15.90%	16.30%