Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,615.9	$ 1,612.3
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	27.8	28.0
Percent of Total	1.70%	1.80%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	504.5	502.0
Percent of Total	31.20%	31.10%
Mixed
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	93.0	93.5
Percent of Total	5.80%	5.80%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	663.8	659.3
Percent of Total	41.10%	40.90%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	319.8	322.4
Percent of Total	19.80%	20.00%
Other
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 7.0	$ 7.1
Percent of Total	0.40%	0.40%