Location of Gains and Losses on Derivative Instruments Designated as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ (22.3)	$ 67.4	$ 16.1
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	53.3	83.4	53.0
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.1)	(0.7)	(0.1)
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(22.2)	68.1	16.2
Investment Income | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	54.5	50.2	47.8
Investment Income | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.1)	(1.6)	(4.2)
Net Realized Investments Gain Loss | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	3.2	0.5	4.3
Net Realized Investments Gain Loss | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.4)	36.1	6.9
Interest Expense | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (1.9)	$ (1.8)	$ (1.8)