Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 1,576.3	$ 1,516.8
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	138.5	146.8
Percent of Total	8.80%	9.70%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	173.4	184.8
Percent of Total	11.00%	12.20%
East North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	215.6	171.7
Percent of Total	13.70%	11.30%
West North Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	145.7	134.6
Percent of Total	9.20%	8.90%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	392.8	372.0
Percent of Total	24.90%	24.50%
East South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	33.6	26.9
Percent of Total	2.10%	1.80%
West South Central
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	159.1	171.8
Percent of Total	10.10%	11.30%
Mountain
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	59.7	60.7
Percent of Total	3.80%	4.00%
Pacific
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 257.9	$ 247.5
Percent of Total	16.40%	16.30%