Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 1,576.3	$ 1,516.8
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	28.3	33.7
Percent of Total	1.80%	2.20%
Industrial
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	480.0	458.2
Percent of Total	30.50%	30.20%
Mixed
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	94.6	95.8
Percent of Total	6.00%	6.30%
Office
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	662.5	634.7
Percent of Total	41.90%	41.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	303.6	286.9
Percent of Total	19.30%	18.90%
Other
Mortgage Loans on Real Estate [Line Items]
Carrying Amount	$ 7.3	$ 7.5
Percent of Total	0.50%	0.50%