Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,434.4	$ 2,397.0
New England
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 41.0	$ 28.9
Mortgage Loans, Percent of Total	1.70%	1.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 206.5	$ 184.5
Mortgage Loans, Percent of Total	8.50%	7.70%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 344.3	$ 329.2
Mortgage Loans, Percent of Total	14.10%	13.70%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 208.0	$ 215.4
Mortgage Loans, Percent of Total	8.50%	9.00%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 517.9	$ 509.2
Mortgage Loans, Percent of Total	21.30%	21.20%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 112.0	$ 114.3
Mortgage Loans, Percent of Total	4.60%	4.80%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 261.4	$ 246.6
Mortgage Loans, Percent of Total	10.70%	10.30%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 254.2	$ 268.2
Mortgage Loans, Percent of Total	10.40%	11.20%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 489.1	$ 500.7
Mortgage Loans, Percent of Total	20.20%	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	$ 641.0	$ 608.8
Mortgage Loans, Percent of Total	26.30%	25.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 609.8	$ 623.6
Mortgage Loans, Percent of Total	25.00%	26.00%
Office
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 533.4	$ 549.3
Mortgage Loans, Percent of Total	21.90%	22.90%
Retail
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 603.2	$ 567.5
Mortgage Loans, Percent of Total	24.90%	23.70%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 47.0	$ 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%