Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,190.6	$ 2,213.2
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 51.2	$ 56.1
Percent of Total	2.30%	2.50%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 153.4	$ 155.5
Percent of Total	7.00%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 307.8	$ 282.0
Percent of Total	14.10%	12.80%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 206.7	$ 210.1
Percent of Total	9.40%	9.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 489.1	$ 494.4
Percent of Total	22.30%	22.30%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 90.1	$ 88.8
Percent of Total	4.10%	4.00%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 215.0	$ 247.4
Percent of Total	9.80%	11.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 253.2	$ 251.2
Percent of Total	11.60%	11.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 424.1	$ 427.7
Percent of Total	19.40%	19.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 394.5	$ 360.0
Percent of Total	18.00%	16.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 617.3	$ 601.2
Percent of Total	28.20%	27.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 604.9	$ 692.3
Percent of Total	27.60%	31.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 542.2	$ 527.6
Percent of Total	24.80%	23.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 31.7	$ 32.1
Percent of Total	1.40%	1.40%