Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 1,925.9	$ 1,856.6
Percent of Total	100.00%	100.00%
New England
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 100.4	$ 105.6
Percent of Total	5.20%	5.70%
Middle Atlantic
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 149.3	$ 179.4
Percent of Total	7.70%	9.70%
East North Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 192.6	$ 210.6
Percent of Total	10.00%	11.40%
West North Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 147.6	$ 166.2
Percent of Total	7.70%	8.90%
South Atlantic
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 448.5	$ 453.6
Percent of Total	23.30%	24.40%
East South Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 79.7	$ 75.3
Percent of Total	4.10%	4.10%
West South Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 232.2	$ 215.6
Percent of Total	12.10%	11.60%
Mountain
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 197.9	$ 116.0
Percent of Total	10.30%	6.20%
Pacific
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 377.7	$ 334.3
Percent of Total	19.60%	18.00%
Apartment
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 103.6	$ 110.1
Percent of Total	5.40%	5.90%
Industrial
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 583.9	$ 542.9
Percent of Total	30.30%	29.20%
Office
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 799.8	$ 794.0
Percent of Total	41.50%	42.80%
Retail
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 416.6	$ 409.6
Percent of Total	21.60%	22.10%
Other Property
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 22.0	$ 0.0
Percent of Total	1.20%	0.00%