Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,038.9	$ 1,883.6
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 72.7	$ 97.6
Percent of Total	3.60%	5.20%
Mid-Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 125.3	$ 128.8
Percent of Total	6.10%	6.90%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 230.1	$ 186.4
Percent of Total	11.30%	9.90%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 172.0	$ 162.6
Percent of Total	8.40%	8.60%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 438.3	$ 409.3
Percent of Total	21.50%	21.70%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 91.6	$ 79.1
Percent of Total	4.50%	4.20%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 268.7	$ 237.6
Percent of Total	13.20%	12.60%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 214.1	$ 196.5
Percent of Total	10.50%	10.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 426.1	$ 385.7
Percent of Total	20.90%	20.50%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 288.4	$ 130.6
Percent of Total	14.10%	6.90%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 573.6	$ 574.1
Percent of Total	28.10%	30.50%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 700.1	$ 764.7
Percent of Total	34.40%	40.60%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 455.4	$ 392.3
Percent of Total	22.40%	20.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 21.4	$ 21.9
Percent of Total	1.00%	1.20%