Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,665.2	$ 1,612.3
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	34.3	28.0
Percent of Total	2.10%	1.80%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	528.4	502.0
Percent of Total	31.70%	31.10%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	78.9	93.5
Percent of Total	4.70%	5.80%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	677.1	659.3
Percent of Total	40.70%	40.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	346.5	322.4
Percent of Total	20.80%	20.00%
Other
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 0	$ 7.1
Percent of Total	0.00%	0.40%