Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,941.3	$ 1,883.6
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 77.4	$ 97.6
Percent of Total	4.00%	5.20%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 126.5	$ 128.8
Percent of Total	6.50%	6.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 205.6	$ 186.4
Percent of Total	10.60%	9.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 167.5	$ 162.6
Percent of Total	8.60%	8.60%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 406.1	$ 409.3
Percent of Total	20.90%	21.70%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 98.6	$ 79.1
Percent of Total	5.10%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 236.1	$ 237.6
Percent of Total	12.20%	12.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 210.9	$ 196.5
Percent of Total	10.90%	10.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 412.6	$ 385.7
Percent of Total	21.20%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 200.7	$ 130.6
Percent of Total	10.40%	6.90%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 580.9	$ 574.1
Percent of Total	29.90%	30.50%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 732.1	$ 764.7
Percent of Total	37.70%	40.60%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 406.0	$ 392.3
Percent of Total	20.90%	20.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 21.6	$ 21.9
Percent of Total	1.10%	1.20%