Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 1,606.9	$ 1,516.8
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	28.1	33.7
Percent of Total	1.70%	2.20%
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	518.2	458.2
Percent of Total	32.30%	30.20%
Mixed
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	94.1	95.8
Percent of Total	5.90%	6.30%
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	658.1	634.7
Percent of Total	41.00%	41.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	301.2	286.9
Percent of Total	18.70%	18.90%
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 7.2	$ 7.5
Percent of Total	0.40%	0.50%