Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,200.8	$ 2,213.2
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 55.2	$ 56.1
Percent of Total	2.50%	2.50%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 154.5	$ 155.5
Percent of Total	7.00%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 291.3	$ 282.0
Percent of Total	13.20%	12.80%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 208.4	$ 210.1
Percent of Total	9.50%	9.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 481.7	$ 494.4
Percent of Total	21.90%	22.30%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 88.1	$ 88.8
Percent of Total	4.00%	4.00%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 245.4	$ 247.4
Percent of Total	11.20%	11.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 254.9	$ 251.2
Percent of Total	11.60%	11.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 421.3	$ 427.7
Percent of Total	19.10%	19.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 380.0	$ 360.0
Percent of Total	17.30%	16.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 584.9	$ 601.2
Percent of Total	26.60%	27.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 674.5	$ 692.3
Percent of Total	30.60%	31.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 529.5	$ 527.6
Percent of Total	24.10%	23.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 31.9	$ 32.1
Percent of Total	1.40%	1.40%