Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,213.2	$ 2,038.9
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 56.1	$ 72.7
Percent of Total	2.50%	3.60%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 155.5	$ 125.3
Percent of Total	7.00%	6.10%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 282.0	$ 230.1
Percent of Total	12.80%	11.30%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 210.1	$ 172.0
Percent of Total	9.50%	8.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 494.4	$ 438.3
Percent of Total	22.30%	21.50%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 88.8	$ 91.6
Percent of Total	4.00%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 247.4	$ 268.7
Percent of Total	11.20%	13.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 251.2	$ 214.1
Percent of Total	11.40%	10.50%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 427.7	$ 426.1
Percent of Total	19.30%	20.90%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 360.0	$ 288.4
Percent of Total	16.30%	14.10%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 601.2	$ 573.6
Percent of Total	27.20%	28.10%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 692.3	$ 700.1
Percent of Total	31.30%	34.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 527.6	$ 455.4
Percent of Total	23.80%	22.40%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 32.1	$ 21.4
Percent of Total	1.40%	1.00%