Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,970.4	$ 1,883.6
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 73.6	$ 97.6
Percent of Total	3.70%	5.20%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 113.7	$ 128.8
Percent of Total	5.80%	6.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 215.3	$ 186.4
Percent of Total	10.90%	9.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 173.6	$ 162.6
Percent of Total	8.80%	8.60%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 417.6	$ 409.3
Percent of Total	21.20%	21.70%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 97.9	$ 79.1
Percent of Total	5.00%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 250.7	$ 237.6
Percent of Total	12.70%	12.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 209.2	$ 196.5
Percent of Total	10.60%	10.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 418.8	$ 385.7
Percent of Total	21.30%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 245.7	$ 130.6
Percent of Total	12.50%	6.90%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 591.5	$ 574.1
Percent of Total	30.00%	30.50%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 701.1	$ 764.7
Percent of Total	35.60%	40.60%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 410.6	$ 392.3
Percent of Total	20.80%	20.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 21.5	$ 21.9
Percent of Total	1.10%	1.20%