Investments Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,218.9	$ 2,295.0
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 41.6	$ 45.9
Percent of Total	1.90%	2.00%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 158.2	$ 160.6
Percent of Total	7.10%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 309.7	$ 354.4
Percent of Total	14.00%	15.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 192.6	$ 190.3
Percent of Total	8.70%	8.30%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 470.5	$ 485.2
Percent of Total	21.20%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 99.7	$ 105.5
Percent of Total	4.50%	4.60%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 219.4	$ 240.6
Percent of Total	9.90%	10.50%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 264.6	$ 242.7
Percent of Total	11.90%	10.60%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 462.6	$ 469.8
Percent of Total	20.80%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 496.2	$ 491.0
Percent of Total	22.40%	21.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 583.7	$ 635.6
Percent of Total	26.30%	27.70%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 577.1	$ 604.2
Percent of Total	26.00%	26.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 513.7	$ 519.5
Percent of Total	23.20%	22.60%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 48.2	$ 44.7
Percent of Total	2.10%	2.00%