Mortgage Loans by Property Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,776.1	$ 1,712.7
Percent of Total	100.00%	100.00%
Apartment
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	70.4	47.2
Percent of Total	4.00%	2.70%
Industrial
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	564.5	545.7
Percent of Total	31.80%	31.90%
Office
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	732.4	721.2
Percent of Total	41.20%	42.10%
Retail
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 408.8	$ 398.6
Percent of Total	23.00%	23.30%