Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,222.0	$ 2,213.2
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 50.3	$ 56.1
Percent of Total	2.30%	2.50%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 161.8	$ 155.5
Percent of Total	7.30%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 343.9	$ 282.0
Percent of Total	15.50%	12.80%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 205.0	$ 210.1
Percent of Total	9.20%	9.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 478.7	$ 494.4
Percent of Total	21.50%	22.30%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 89.4	$ 88.8
Percent of Total	4.00%	4.00%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 213.3	$ 247.4
Percent of Total	9.60%	11.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 243.0	$ 251.2
Percent of Total	10.90%	11.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 436.6	$ 427.7
Percent of Total	19.70%	19.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 428.9	$ 360.0
Percent of Total	19.30%	16.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 626.5	$ 601.2
Percent of Total	28.20%	27.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 587.8	$ 692.3
Percent of Total	26.50%	31.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 533.9	$ 527.6
Percent of Total	24.00%	23.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 44.9	$ 32.1
Percent of Total	2.00%	1.40%