Mortgage Loans by Property Type and Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Mortgage Loans on Real Estate
Mortgage Loans	$ 1,807.5	$ 1,815.1
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	85.6	61.1
Percent of Total	4.70%	3.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	522.1	567.8
Percent of Total	28.90%	31.30%
Office
Mortgage Loans on Real Estate
Mortgage Loans	792.1	776.5
Percent of Total	43.80%	42.80%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	407.7	409.7
Percent of Total	22.60%	22.60%
New England
Mortgage Loans on Real Estate
Mortgage Loans	111.9	100.9
Percent of Total	6.20%	5.60%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	180.9	191.5
Percent of Total	10.00%	10.50%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	230.4	244.3
Percent of Total	12.70%	13.50%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	168.0	162.3
Percent of Total	9.30%	8.90%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	445.1	447.7
Percent of Total	24.60%	24.70%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	65.7	67.7
Percent of Total	3.60%	3.70%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	200.4	190.9
Percent of Total	11.10%	10.50%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	107.5	101.9
Percent of Total	6.00%	5.60%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 297.6	$ 307.9
Percent of Total	16.50%	17.00%