Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,856.5	$ 1,856.6
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 103.1	$ 105.6
Percent of Total	5.60%	5.70%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 161.5	$ 179.4
Percent of Total	8.70%	9.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 195.7	$ 210.6
Percent of Total	10.60%	11.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 156.6	$ 166.2
Percent of Total	8.40%	8.90%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 440.4	$ 453.6
Percent of Total	23.70%	24.40%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 80.2	$ 75.3
Percent of Total	4.30%	4.10%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 238.2	$ 215.6
Percent of Total	12.80%	11.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 164.0	$ 116.0
Percent of Total	8.80%	6.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 316.8	$ 334.3
Percent of Total	17.10%	18.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 116.8	$ 110.1
Percent of Total	6.30%	5.90%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 548.0	$ 542.9
Percent of Total	29.50%	29.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 769.7	$ 794.0
Percent of Total	41.50%	42.80%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 422.0	$ 409.6
Percent of Total	22.70%	22.10%