Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,652.5	$ 1,612.3
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	134.8	147.0
Percent of Total	8.20%	9.10%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	162.5	174.1
Percent of Total	9.80%	10.80%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	209.9	212.7
Percent of Total	12.70%	13.20%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	148.6	151.2
Percent of Total	9.00%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	404.4	383.8
Percent of Total	24.40%	23.80%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	80.6	52.4
Percent of Total	4.90%	3.30%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	174.8	160.4
Percent of Total	10.60%	9.90%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	76.1	69.5
Percent of Total	4.60%	4.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 260.8	$ 261.2
Percent of Total	15.80%	16.20%