Location of Gains and Losses on Derivative Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 16.1	$ 15.3	$ 81.4
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	53.0	23.0	20.1
Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.1)	(7.2)	77.9
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	47.8	43.1	40.0
Interest Rate Swaps | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	4.3	1.3	4.1
Interest Rate Swaps | Interest Expense
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.8)	(1.7)	(1.7)
Options Held
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0	(0.1)	0
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	16.2	22.6	3.5
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(4.2)	(5.9)	(5.3)
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 6.9	$ (13.8)	$ (17.0)