Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,397.0	$ 2,295.0
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 28.9	$ 45.9
Percent of Total	1.20%	2.00%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 184.5	$ 160.6
Percent of Total	7.70%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 329.2	$ 354.4
Percent of Total	13.70%	15.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 215.4	$ 190.3
Percent of Total	9.00%	8.30%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 509.2	$ 485.2
Percent of Total	21.20%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 114.3	$ 105.5
Percent of Total	4.80%	4.60%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 246.6	$ 240.6
Percent of Total	10.30%	10.50%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 268.2	$ 242.7
Percent of Total	11.20%	10.60%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 500.7	$ 469.8
Percent of Total	20.90%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 608.8	$ 491.0
Percent of Total	25.40%	21.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 623.6	$ 635.6
Percent of Total	26.00%	27.70%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 549.3	$ 604.2
Percent of Total	22.90%	26.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 567.5	$ 519.5
Percent of Total	23.70%	22.60%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 47.8	$ 44.7
Percent of Total	2.00%	2.00%