Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,145.3	$ 2,038.9
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 57.0	$ 72.7
Percent of Total	2.70%	3.60%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 156.5	$ 125.3
Percent of Total	7.30%	6.10%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 262.6	$ 230.1
Percent of Total	12.20%	11.30%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 211.8	$ 172.0
Percent of Total	9.90%	8.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 459.9	$ 438.3
Percent of Total	21.40%	21.50%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 89.5	$ 91.6
Percent of Total	4.20%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 263.3	$ 268.7
Percent of Total	12.30%	13.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 226.0	$ 214.1
Percent of Total	10.50%	10.50%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 418.7	$ 426.1
Percent of Total	19.50%	20.90%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 361.9	$ 288.4
Percent of Total	16.90%	14.10%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 560.2	$ 573.6
Percent of Total	26.10%	28.10%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 691.8	$ 700.1
Percent of Total	32.20%	34.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 499.1	$ 455.4
Percent of Total	23.30%	22.40%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 32.3	$ 21.4
Percent of Total	1.50%	1.00%