Investments (Schedule Of Aging Of Unrealized Losses On Investments) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 288.3
Unrealized Losses, Less Than One Year	14.0
Fair Value, One Year or Greater	45.2
Unrealized Losses, One Year or Greater	6.3
Fair Value, Total	333.5		651.0	[1]
Unrealized Losses, Total	20.3		38.1	[1]
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	232.1
Unrealized Losses, Less Than One Year	9.1
Fair Value, One Year or Greater	31.2
Unrealized Losses, One Year or Greater	3.1
Fair Value, Total	263.3		546.3
Unrealized Losses, Total	12.2		28.5
U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	2.2	[1],[2]
Unrealized Losses, Less Than One Year	0	[1],[2]
Fair Value, One Year or Greater	0	[1],[2]
Unrealized Losses, One Year or Greater	0	[1],[2]
Fair Value, Total	2.2	[1],[2]	41.0	[1]
Unrealized Losses, Total	0	[1],[2]	0	[1]
Non-U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	5.4	[2]
Unrealized Losses, Less Than One Year	0.1	[2]
Fair Value, One Year or Greater	0.6	[2]
Unrealized Losses, One Year or Greater	0	[2]
Fair Value, Total	6.0	[2]	22.4
Unrealized Losses, Total	0.1	[2]	1.6
Obligations Of States And Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	5.1
Unrealized Losses, Less Than One Year	0.1
Fair Value, One Year or Greater	0.7
Unrealized Losses, One Year or Greater	0.2
Fair Value, Total	5.8		2.1	[1]
Unrealized Losses, Total	0.3		0.3	[1]
Credit-Financial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	10.2
Unrealized Losses, Less Than One Year	0.1
Fair Value, One Year or Greater	9.9
Unrealized Losses, One Year or Greater	0.6
Fair Value, Total	20.1		91.2
Unrealized Losses, Total	0.7		5.2
Credit-Industrial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	21.6
Unrealized Losses, Less Than One Year	0.4
Fair Value, One Year or Greater	6.2
Unrealized Losses, One Year or Greater	0.6
Fair Value, Total	27.8		67.4
Unrealized Losses, Total	1.0		3.4
Credit-Utility [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	4.4
Unrealized Losses, Less Than One Year	0.2
Fair Value, One Year or Greater	1.3
Unrealized Losses, One Year or Greater	0.4
Fair Value, Total	5.7		21.7
Unrealized Losses, Total	0.6		1.2
CMO/MBS-Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	8.4	[2]
Unrealized Losses, Less Than One Year	0.1	[2]
Fair Value, One Year or Greater	0.3	[2]
Unrealized Losses, One Year or Greater	0	[2]
Fair Value, Total	8.7	[2]	32.1
Unrealized Losses, Total	0.1	[2]	0.2
CMO/MBS-Non Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	2.0
Unrealized Losses, Less Than One Year	0.1
Fair Value, One Year or Greater	2.5
Unrealized Losses, One Year or Greater	0.1
Fair Value, Total	4.5		10.9
Unrealized Losses, Total	0.2		0.8
CMBS [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0.4	[1]
Unrealized Losses, Less Than One Year	0	[1]
Fair Value, One Year or Greater	3.8	[1]
Unrealized Losses, One Year or Greater	0.2	[1]
Fair Value, Total	4.2	[1]	4.9	[1]
Unrealized Losses, Total	0.2	[1]	0.3	[1]
ABS-Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0.3	[1]
Unrealized Losses, Less Than One Year	0	[1]
Fair Value, One Year or Greater	5.4	[1]
Unrealized Losses, One Year or Greater	1.0	[1]
Fair Value, Total	5.7	[1]	10.8
Unrealized Losses, Total	1.0	[1]	2.0
ABS-Non Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	3.9	[1],[2]
Unrealized Losses, Less Than One Year	0	[1],[2]
Fair Value, One Year or Greater	0.2	[1],[2]
Unrealized Losses, One Year or Greater	0	[1],[2]
Fair Value, Total	4.1	[1],[2]	4.3	[1],[2]
Unrealized Losses, Total	0	[1],[2]	0	[1],[2]
Foreign Denominated, Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	118.9	[2]
Unrealized Losses, Less Than One Year	5.3	[2]
Fair Value, One Year or Greater	0.3	[2]
Unrealized Losses, One Year or Greater	0	[2]
Fair Value, Total	119.2	[2]	166.3
Unrealized Losses, Total	5.3	[2]	8.8
Foreign Denominated, Credit [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	49.3
Unrealized Losses, Less Than One Year	2.7
Fair Value, One Year or Greater	0
Unrealized Losses, One Year or Greater	0
Fair Value, Total	49.3		71.2
Unrealized Losses, Total	2.7		4.7
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	40.8
Unrealized Losses, Less Than One Year	2.7
Fair Value, One Year or Greater	14.0
Unrealized Losses, One Year or Greater	3.2
Fair Value, Total	54.8		84.5
Unrealized Losses, Total	5.9		8.6
Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	14.3
Unrealized Losses, Less Than One Year	2.2
Fair Value, One Year or Greater	0
Unrealized Losses, One Year or Greater	0
Fair Value, Total	14.3		(1.0)	[3]
Unrealized Losses, Total	2.2		1.0	[3]
Short-term Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1.1
Unrealized Losses, Less Than One Year	0
Fair Value, One Year or Greater	0
Unrealized Losses, One Year or Greater	0
Fair Value, Total	1.1		21.2
Unrealized Losses, Total	0		0
Less Than One Year [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			613.9	[1]
Unrealized Losses, Less Than One Year			32.8	[1]
Less Than One Year [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			509.4
Unrealized Losses, Less Than One Year			23.3
Less Than One Year [Member] | U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			41.0	[1]
Unrealized Losses, Less Than One Year			0	[1]
Less Than One Year [Member] | Non-U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			22.4
Unrealized Losses, Less Than One Year			1.6
Less Than One Year [Member] | Obligations Of States And Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			1.4	[1]
Unrealized Losses, Less Than One Year			0	[1]
Less Than One Year [Member] | Credit-Financial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			81.8
Unrealized Losses, Less Than One Year			3.8
Less Than One Year [Member] | Credit-Industrial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			67.4
Unrealized Losses, Less Than One Year			3.4
Less Than One Year [Member] | Credit-Utility [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			21.7
Unrealized Losses, Less Than One Year			1.2
Less Than One Year [Member] | CMO/MBS-Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			28.9
Unrealized Losses, Less Than One Year			0.1
Less Than One Year [Member] | CMO/MBS-Non Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			6.7
Unrealized Losses, Less Than One Year			0.2
Less Than One Year [Member] | CMBS [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			0.7	[1]
Unrealized Losses, Less Than One Year			0	[1]
Less Than One Year [Member] | ABS-Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			1.2
Unrealized Losses, Less Than One Year			0.1
Less Than One Year [Member] | ABS-Non Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			4.0	[1],[2]
Unrealized Losses, Less Than One Year			0	[1],[2]
Less Than One Year [Member] | Foreign Denominated, Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			166.3
Unrealized Losses, Less Than One Year			8.8
Less Than One Year [Member] | Foreign Denominated, Credit [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			65.9
Unrealized Losses, Less Than One Year			4.1
Less Than One Year [Member] | Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			84.3
Unrealized Losses, Less Than One Year			8.5
Less Than One Year [Member] | Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			(1.0)	[3]
Unrealized Losses, Less Than One Year			1.0	[3]
Less Than One Year [Member] | Short-term Investments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year			21.2
Unrealized Losses, Less Than One Year			0
One Year Or Greater [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			37.1	[1]
Unrealized Losses, One Year or Greater			5.3	[1]
One Year Or Greater [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			36.9
Unrealized Losses, One Year or Greater			5.2
One Year Or Greater [Member] | U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0	[1]
Unrealized Losses, One Year or Greater			0	[1]
One Year Or Greater [Member] | Non-U.S. Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0
Unrealized Losses, One Year or Greater			0
One Year Or Greater [Member] | Obligations Of States And Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0.7	[1]
Unrealized Losses, One Year or Greater			0.3	[1]
One Year Or Greater [Member] | Credit-Financial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			9.4
Unrealized Losses, One Year or Greater			1.4
One Year Or Greater [Member] | Credit-Industrial [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0
Unrealized Losses, One Year or Greater			0
One Year Or Greater [Member] | Credit-Utility [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0
Unrealized Losses, One Year or Greater			0
One Year Or Greater [Member] | CMO/MBS-Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			3.2
Unrealized Losses, One Year or Greater			0.1
One Year Or Greater [Member] | CMO/MBS-Non Agency [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			4.2
Unrealized Losses, One Year or Greater			0.6
One Year Or Greater [Member] | CMBS [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			4.2	[1]
Unrealized Losses, One Year or Greater			0.3	[1]
One Year Or Greater [Member] | ABS-Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			9.6
Unrealized Losses, One Year or Greater			1.9
One Year Or Greater [Member] | ABS-Non Residential [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0.3	[1],[2]
Unrealized Losses, One Year or Greater			0	[1],[2]
One Year Or Greater [Member] | Foreign Denominated, Governments [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0
Unrealized Losses, One Year or Greater			0
One Year Or Greater [Member] | Foreign Denominated, Credit [Member] | Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			5.3
Unrealized Losses, One Year or Greater			0.6
One Year Or Greater [Member] | Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0.2
Unrealized Losses, One Year or Greater			0.1
One Year Or Greater [Member] | Other Investments [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0	[3]
Unrealized Losses, One Year or Greater			0	[3]
One Year Or Greater [Member] | Short-term Investments [Member]
Schedule of Investments [Line Items]
Fair Value, One Year or Greater			0
Unrealized Losses, One Year or Greater			$ 0

[1] Unrealized losses less than one year are less than $0.1 million.

[2] Unrealized losses one year or greater are less than $0.1 million.

[3] Unrealized foreign currency exchange position at December 31, 2011.