Location of Gains and Losses on Derivative Instruments Designated as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ (23.1)	$ (22.3)	$ 67.4
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	46.8	53.3	83.4
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.1)	(0.1)	(0.7)
Interest Rate Swaps | Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	59.0	54.5	50.2
Interest Rate Swaps | Net Realized Investments Gain Loss
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.2	3.2	0.5
Interest Rate Swaps | Interest Expense
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(2.0)	(1.9)	(1.8)
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(23.0)	(22.2)	68.1
Foreign Exchange Contracts | Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.2)	(1.1)	(1.6)
Foreign Exchange Contracts | Net Realized Investments Gain Loss
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (9.2)	$ (1.4)	$ 36.1