Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,883.6	$ 1,856.6
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 130.6	$ 110.1
Percent of Total	6.90%	5.90%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 574.1	$ 542.9
Percent of Total	30.50%	29.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 764.7	$ 794.0
Percent of Total	40.60%	42.80%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 392.3	$ 409.6
Percent of Total	20.80%	22.10%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 21.9	$ 0.0
Percent of Total	1.20%	0.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 97.6	$ 105.6
Percent of Total	5.20%	5.70%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 128.8	$ 179.4
Percent of Total	6.90%	9.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 186.4	$ 210.6
Percent of Total	9.90%	11.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 162.6	$ 166.2
Percent of Total	8.60%	8.90%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 409.3	$ 453.6
Percent of Total	21.70%	24.40%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 79.1	$ 75.3
Percent of Total	4.20%	4.10%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 237.6	$ 215.6
Percent of Total	12.60%	11.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 196.5	$ 116.0
Percent of Total	10.40%	6.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 385.7	$ 334.3
Percent of Total	20.50%	18.00%