Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2012 Debt Securities Mortgage and asset backed securities CMBS	Sep. 30, 2012 Debt Securities Mortgage and asset backed securities CMBS		Jan. 02, 2012 Debt Securities U.S. Government obligations
Fair Value, Measurement Inputs, Disclosure [Line Items]
Gross transfers out of Level 2 into Level 3	$ 41.9	[1]	$ 41.9	$ 41.9	[1]
Gross transfers out of Level 1 into Level 2						$ 267.8

[1] Principally due to a decrease in observable inputs related to the valuation of such securities.