Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Derivative [Line Items]
Balance Sheet - Fair Value	$ 53.5	[1]	$ (101.3)	[1]	$ 53.5	[1]	$ (101.3)	[1]	$ (95.5)	[1]
Income Statement - Net Realized Gain (Loss) on Securities	(1.2)		(16.3)		45.0		(42.8)
Designated as Hedging Instruments | Closed Positions | Ineffective Cash Flow Hedge
Derivative [Line Items]
Notional Value	54.0	[2]	31.0	[2]	54.0	[2]	31.0	[2]	31.0	[2]
Purpose					Manage interest  rate risk
Balance Sheet - Fair Value	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Income Statement - Net Realized Gain (Loss) on Securities	0.8		0		0.8		0.6
Non-hedging Instruments | Closed Positions | Interest rate swaps
Derivative [Line Items]
Notional Value	1,263.0	[2]	0	[2]	1,263.0	[2]	0	[2]	0	[2]
Purpose					Manage portfolio duration
Balance Sheet - Fair Value	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Income Statement - Net Realized Gain (Loss) on Securities	0		0		(4.0)		0
Non-hedging Instruments | Closed Positions | Corporate Credit Default Swaps
Derivative [Line Items]
Notional Value	0	[2]	0	[2]	0	[2]	0	[2]	25.0	[2]
Purpose					Manage credit risk
Balance Sheet - Fair Value	0	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Income Statement - Net Realized Gain (Loss) on Securities	0		0		0		0
Non-hedging Instruments | Assets | Interest rate swaps | Fixed maturities
Derivative [Line Items]
Notional Value	750.0	[2]	0	[2]	750.0	[2]	0	[2]	0	[2]
Purpose					Manage portfolio duration
Balance Sheet - Fair Value	53.5	[1]	0	[1]	53.5	[1]	0	[1]	0	[1]
Income Statement - Net Realized Gain (Loss) on Securities	(2.0)		0		48.2		0
Non-hedging Instruments | Assets | Corporate Credit Default Swaps | Fixed maturities
Derivative [Line Items]
Notional Value	0	[2]	25.0	[2]	0	[2]	25.0	[2]	0	[2]
Purpose					Manage credit risk
Balance Sheet - Fair Value	0	[1]	0.3	[1]	0	[1]	0.3	[1]	0	[1]
Income Statement - Net Realized Gain (Loss) on Securities	0		(0.4)		0		(0.7)
Non-hedging Instruments | Liabilities | Interest rate swaps | Other Liabilities
Derivative [Line Items]
Notional Value	0	[2]	1,263.0	[2]	0	[2]	1,263.0	[2]	1,263.0	[2]
Purpose					Manage portfolio duration
Balance Sheet - Fair Value	0	[1]	(101.6)	[1]	0	[1]	(101.6)	[1]	(95.5)	[1]
Income Statement - Net Realized Gain (Loss) on Securities	$ 0		$ (15.9)		$ 0		$ (42.7)

[1] To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we expect that we will report them on a gross basis on our balance sheets, consistent with our historical presentation.

[2] The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions.