Commitments and Contingent Liabilities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2011	Mar. 31, 2011
Commitments and Contingencies Disclosure [Line Items]
Operating Leases, Future Minimum Payments Due	$ 216.8
Operating Leases, Future Minimum Payments Due, Current	33.7
Operating Leases, Future Minimum Payments, Due in Two Years	30.6
Operating Leases, Future Minimum Payments, Due in Three Years	23.4
Operating Leases, Future Minimum Payments, Due in Four Years	17.1
Operating Leases, Future Minimum Payments, Due in Five Years	14.0
Operating Leases, Future Minimum Payments, Due Thereafter	98.0
Operating Leases, Rent Expense, Minimum Rentals	41.6	36.1	29.3
Commitment to fund partnership Equity Investments	71.3
Commitment to fund private placement fixed maturity securities	29.0
Committments to Fund Commercial Mortgage Loans	$ 47.3
Interest on undrawn retained asset account funds			18.00%
States involved in unclaimed property examination					32
Insurers requested to cross-check benefits				160