Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,730.4	$ 1,712.7
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	33.8	34.0
Percent of Total	2.00%	2.00%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	518.0	513.4
Percent of Total	29.90%	30.00%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	77.8	78.4
Percent of Total	4.50%	4.60%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	730.6	713.4
Percent of Total	42.20%	41.60%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 370.2	$ 373.5
Percent of Total	21.40%	21.80%