Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,615.9	$ 1,612.3
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	145.8	147.0
Percent of Total	9.00%	9.10%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	172.5	174.1
Percent of Total	10.70%	10.80%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	211.3	212.7
Percent of Total	13.10%	13.20%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	149.9	151.2
Percent of Total	9.30%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	397.5	383.8
Percent of Total	24.60%	23.80%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	52.1	52.4
Percent of Total	3.20%	3.30%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	158.8	160.4
Percent of Total	9.80%	9.90%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	69.0	69.5
Percent of Total	4.30%	4.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 259.0	$ 261.2
Percent of Total	16.00%	16.20%