Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,652.5	$ 1,612.3
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	34.5	28.0
Percent of Total	2.10%	1.80%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	519.3	502.0
Percent of Total	31.40%	31.10%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	79.4	93.5
Percent of Total	4.80%	5.80%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	677.6	659.3
Percent of Total	41.00%	40.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	334.7	322.4
Percent of Total	20.30%	20.00%
Other
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 7.0	$ 7.1
Percent of Total	0.40%	0.40%