Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,295.0	$ 2,213.2
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 45.9	$ 56.1
Percent of Total	2.00%	2.50%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 160.6	$ 155.5
Percent of Total	7.00%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 354.4	$ 282.0
Percent of Total	15.40%	12.80%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 190.3	$ 210.1
Percent of Total	8.30%	9.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 485.2	$ 494.4
Percent of Total	21.10%	22.30%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 105.5	$ 88.8
Percent of Total	4.60%	4.00%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 240.6	$ 247.4
Percent of Total	10.50%	11.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 242.7	$ 251.2
Percent of Total	10.60%	11.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 469.8	$ 427.7
Percent of Total	20.50%	19.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 491.0	$ 360.0
Percent of Total	21.40%	16.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 635.6	$ 601.2
Percent of Total	27.70%	27.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 604.2	$ 692.3
Percent of Total	26.30%	31.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 519.5	$ 527.6
Percent of Total	22.60%	23.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 44.7	$ 32.1
Percent of Total	2.00%	1.40%