Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 0.3	$ 32.7	$ (24.2)	$ 59.1
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	13.0	12.1	41.1	36.2
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.0	(0.8)	0.0	(0.8)
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	13.8	12.8	40.6	38.3
Interest Rate Swaps | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.0	0.3	3.2	0.6
Interest Rate Swaps | Interest Expense
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.5)	(0.5)	(1.4)	(1.4)
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0.3	33.5	(24.2)	59.9
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.3)	(0.3)	(0.8)	(1.3)
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 0.0	$ (0.2)	$ (0.5)	$ 0.0