Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,068.4	$ 2,038.9
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 71.8	$ 72.7
Percent of Total	3.50%	3.60%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 120.1	$ 125.3
Percent of Total	5.80%	6.10%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 233.4	$ 230.1
Percent of Total	11.30%	11.30%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 179.1	$ 172.0
Percent of Total	8.60%	8.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 440.8	$ 438.3
Percent of Total	21.30%	21.50%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 90.9	$ 91.6
Percent of Total	4.40%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 262.2	$ 268.7
Percent of Total	12.70%	13.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 235.3	$ 214.1
Percent of Total	11.40%	10.50%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 434.8	$ 426.1
Percent of Total	21.00%	20.90%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 291.8	$ 288.4
Percent of Total	14.10%	14.10%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 580.8	$ 573.6
Percent of Total	28.10%	28.10%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 712.7	$ 700.1
Percent of Total	34.40%	34.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 450.4	$ 455.4
Percent of Total	21.80%	22.40%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 32.7	$ 21.4
Percent of Total	1.60%	1.00%