Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,206.6	$ 2,295.0
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 45.0	$ 45.9
Percent of Total	2.00%	2.00%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 159.4	$ 160.6
Percent of Total	7.20%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 311.7	$ 354.4
Percent of Total	14.10%	15.40%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 188.7	$ 190.3
Percent of Total	8.60%	8.30%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 481.2	$ 485.2
Percent of Total	21.80%	21.10%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 100.5	$ 105.5
Percent of Total	4.60%	4.60%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 213.0	$ 240.6
Percent of Total	9.70%	10.50%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 254.1	$ 242.7
Percent of Total	11.50%	10.60%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 453.0	$ 469.8
Percent of Total	20.50%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 471.4	$ 491.0
Percent of Total	21.40%	21.40%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 588.0	$ 635.6
Percent of Total	26.60%	27.70%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 585.0	$ 604.2
Percent of Total	26.50%	26.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 517.8	$ 519.5
Percent of Total	23.50%	22.60%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 44.4	$ 44.7
Percent of Total	2.00%	2.00%