Investments Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,321.7	$ 2,295.0
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 40.7	$ 45.9
Percent of Total	1.80%	2.00%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 170.0	$ 160.6
Percent of Total	7.30%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 290.1	$ 354.4
Percent of Total	12.50%	15.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 217.3	$ 190.3
Percent of Total	9.30%	8.30%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 527.2	$ 485.2
Percent of Total	22.70%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 115.2	$ 105.5
Percent of Total	5.00%	4.60%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 222.3	$ 240.6
Percent of Total	9.60%	10.50%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 262.9	$ 242.7
Percent of Total	11.30%	10.60%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 476.0	$ 469.8
Percent of Total	20.50%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 535.4	$ 491.0
Percent of Total	23.10%	21.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 608.1	$ 635.6
Percent of Total	26.20%	27.70%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 561.5	$ 604.2
Percent of Total	24.20%	26.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 568.7	$ 519.5
Percent of Total	24.50%	22.60%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 48.0	$ 44.7
Percent of Total	2.00%	2.00%