Investments Derivative Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Derivative [Line Items]
Balance Sheet - Fair Value	$ 35.1	[1]	$ 52.4	[1]	$ 35.1	[1]	$ 52.4	[1]	$ 68.1	[1]
Income Statement - Net Realized Gain (Loss) on Securities	(19.9)		44.9		(39.2)		46.2
Designated as Hedging Instruments | Closed Positions | Ineffective Cash Flow Hedge
Derivative [Line Items]
Notional Value	0	[2]	0	[2]	0	[2]	0	[2]	54	[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		0		0		0
Non-hedging Instruments | Closed Positions | Interest rate swaps
Derivative [Line Items]
Notional Value	0	[2]	1,263	[2]	0	[2]	1,263	[2]	1,263	[2]
Purpose							Manage portfolio duration		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		(5.3)		0		(4.0)
Non-hedging Instruments | Assets | Interest rate swaps | Fixed maturities
Derivative [Line Items]
Notional Value	750	[2]	750	[2]	750	[2]	750	[2]	750	[2]
Purpose					Manage portfolio duration		Manage portfolio duration		Manage portfolio duration
Balance Sheet - Fair Value	35.1	[1]	52.4	[1]	35.1	[1]	52.4	[1]	68.1	[1]
Income Statement - Net Realized Gain (Loss) on Securities	$ (19.9)		$ 50.2		$ (39.2)		$ 50.2

[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 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.