Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,499.4	$ 2,432.1
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 55.4	$ 40.0
Mortgage Loans, Percent of Total	2.20%	1.60%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 196.4	$ 202.5
Mortgage Loans, Percent of Total	7.90%	8.20%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 315.2	$ 330.4
Mortgage Loans, Percent of Total	12.60%	13.60%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 190.6	$ 196.1
Mortgage Loans, Percent of Total	7.60%	8.10%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 570.1	$ 512.0
Mortgage Loans, Percent of Total	22.80%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 113.6	$ 110.0
Mortgage Loans, Percent of Total	4.50%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 245.1	$ 257.4
Mortgage Loans, Percent of Total	9.90%	10.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 278.1	$ 268.8
Mortgage Loans, Percent of Total	11.10%	11.10%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 534.9	$ 514.9
Mortgage Loans, Percent of Total	21.40%	21.20%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 715.8	$ 638.0
Mortgage Loans, Percent of Total	28.60%	26.20%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 740.2	$ 654.0
Mortgage Loans, Percent of Total	29.70%	26.90%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 472.1	$ 517.8
Mortgage Loans, Percent of Total	18.90%	21.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 525.5	$ 575.6
Mortgage Loans, Percent of Total	21.00%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 45.8	$ 46.7
Mortgage Loans, Percent of Total	1.80%	1.90%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%