Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,081.2	$ 2,038.9
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 70.8	$ 72.7
Percent of Total	3.40%	3.60%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 127.6	$ 125.3
Percent of Total	6.10%	6.10%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 249.4	$ 230.1
Percent of Total	12.00%	11.30%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 188.9	$ 172.0
Percent of Total	9.10%	8.40%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 435.7	$ 438.3
Percent of Total	20.90%	21.50%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 90.2	$ 91.6
Percent of Total	4.30%	4.50%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 260.3	$ 268.7
Percent of Total	12.50%	13.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 229.8	$ 214.1
Percent of Total	11.10%	10.50%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 428.5	$ 426.1
Percent of Total	20.60%	20.90%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 309.0	$ 288.4
Percent of Total	14.80%	14.10%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 569.5	$ 573.6
Percent of Total	27.30%	28.10%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 702.5	$ 700.1
Percent of Total	33.80%	34.40%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 467.7	$ 455.4
Percent of Total	22.50%	22.40%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 32.5	$ 21.4
Percent of Total	1.60%	1.00%