Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,735.3	$ 1,712.7
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	53.1	34.0
Percent of Total	3.10%	2.00%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	510.5	513.4
Percent of Total	29.40%	30.00%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	77.3	78.4
Percent of Total	4.40%	4.60%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	727.1	713.4
Percent of Total	41.90%	41.60%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 367.3	$ 373.5
Percent of Total	21.20%	21.80%