Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,406.5	$ 2,397.0
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 40.5	$ 28.9
Mortgage Loans, Percent of Total	1.70%	1.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 204.4	$ 184.5
Mortgage Loans, Percent of Total	8.50%	7.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 341.8	$ 329.2
Mortgage Loans, Percent of Total	14.20%	13.70%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 201.6	$ 215.4
Mortgage Loans, Percent of Total	8.40%	9.00%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 509.6	$ 509.2
Mortgage Loans, Percent of Total	21.20%	21.20%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 110.9	$ 114.3
Mortgage Loans, Percent of Total	4.60%	4.80%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 259.0	$ 246.6
Mortgage Loans, Percent of Total	10.80%	10.30%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 252.6	$ 268.2
Mortgage Loans, Percent of Total	10.50%	11.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 486.1	$ 500.7
Mortgage Loans, Percent of Total	20.10%	20.90%
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	$ 635.6	$ 608.8
Mortgage Loans, Percent of Total	26.40%	25.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 602.7	$ 623.6
Mortgage Loans, Percent of Total	25.10%	26.00%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 522.8	$ 549.3
Mortgage Loans, Percent of Total	21.70%	22.90%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 598.6	$ 567.5
Mortgage Loans, Percent of Total	24.90%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 46.8	$ 47.8
Mortgage Loans, Percent of Total	1.90%	2.00%
Total
Mortgage Loans on Real Estate
Mortgage Loans, Percent of Total	100.00%	100.00%