Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,402.2	$ 2,432.1
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 35.1	$ 40.0
Mortgage Loans, Percent of Total	1.50%	1.60%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 200.6	$ 202.5
Mortgage Loans, Percent of Total	8.40%	8.20%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 310.4	$ 330.4
Mortgage Loans, Percent of Total	12.90%	13.60%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 194.3	$ 196.1
Mortgage Loans, Percent of Total	8.10%	8.10%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 510.4	$ 512.0
Mortgage Loans, Percent of Total	21.20%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 109.1	$ 110.0
Mortgage Loans, Percent of Total	4.50%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 255.3	$ 257.4
Mortgage Loans, Percent of Total	10.60%	10.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 276.3	$ 268.8
Mortgage Loans, Percent of Total	11.50%	11.10%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 510.7	$ 514.9
Mortgage Loans, Percent of Total	21.30%	21.20%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100000000.00%	100000000.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 648.2	$ 638.0
Mortgage Loans, Percent of Total	27.00%	26.20%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 650.6	$ 654.0
Mortgage Loans, Percent of Total	27.10%	26.90%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 490.8	$ 517.8
Mortgage Loans, Percent of Total	20.40%	21.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 566.1	$ 575.6
Mortgage Loans, Percent of Total	23.60%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 46.5	$ 46.7
Mortgage Loans, Percent of Total	1.90%	1.90%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%