Types of Risk Management Derivative Instruments Not Formally Designated as Hedges and Their Notional Amounts and Fair Values (Detail) (Nondesignated, USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Notional Value	$ 189.0	$ 1,232.3
Fair Value Asset	0.4	10.3
Fair Value Liability	3.9	4.0
Credit Default Swap Contracts
Derivative [Line Items]
Notional Value	25.0	42.5
Fair Value Liability	0.1	1.0
Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	164.0	1,189.8
Fair Value Asset	0.4	10.3
Fair Value Liability	$ 3.8	$ 3.0