Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 1,881.3	$ 1,883.6
Percent of Total	100.00%	100.00%
New England
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 81.1	$ 97.6
Percent of Total	4.30%	5.20%
Middle Atlantic
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 127.6	$ 128.8
Percent of Total	6.80%	6.90%
East North Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 194.8	$ 186.4
Percent of Total	10.30%	9.90%
West North Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 147.0	$ 162.6
Percent of Total	7.80%	8.60%
South Atlantic
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 401.7	$ 409.3
Percent of Total	21.40%	21.70%
East South Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 91.3	$ 79.1
Percent of Total	4.90%	4.20%
West South Central
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 230.3	$ 237.6
Percent of Total	12.20%	12.60%
Mountain
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 194.9	$ 196.5
Percent of Total	10.40%	10.40%
Pacific
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 412.6	$ 385.7
Percent of Total	21.90%	20.50%
Apartment
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 164.1	$ 130.6
Percent of Total	8.70%	6.90%
Industrial
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 570.1	$ 574.1
Percent of Total	30.30%	30.50%
Office
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 741.7	$ 764.7
Percent of Total	39.40%	40.60%
Retail
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 383.6	$ 392.3
Percent of Total	20.40%	20.80%
Other Property
Disclosure Mortgage Loans by Geographic Region [Abstract]
Mortgage Loans	$ 21.8	$ 21.9
Percent of Total	1.20%	1.20%