Investments (Schedule Of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses And Fair Value Of Investments) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,012.0		$ 3,915.6
d losses and fair 31, 2011 were as Gross Unrealized Gains	353.2		270.0
Gross Unrealized Losses	20.3		38.1
Fair value	4,344.9		4,147.5
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	373.3		291.5
d losses and fair 31, 2011 were as Gross Unrealized Gains	165.9		120.7
Gross Unrealized Losses	5.9		8.6
Fair value	533.3		403.6
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	282.7		232.3
d losses and fair 31, 2011 were as Gross Unrealized Gains	3.1		0.7
Gross Unrealized Losses	2.2		1.0
Fair value	283.6		232.0
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	370.5		296.4
d losses and fair 31, 2011 were as Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair value	370.5		296.4
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,985.5		3,095.4
d losses and fair 31, 2011 were as Gross Unrealized Gains	184.2		148.6
Gross Unrealized Losses	12.2		28.5
Fair value	3,157.5		3,215.5
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	406.0	[1]	482.8	[1],[2]
d losses and fair 31, 2011 were as Gross Unrealized Gains	10.9	[1]	12.5	[1],[2]
Gross Unrealized Losses	0	[1]	0	[1],[2]
Fair value	416.9	[1]	495.3	[1],[2]
Fixed Maturities [Member] | Non-U.S. Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	56.6		56.2
d losses and fair 31, 2011 were as Gross Unrealized Gains	3.5		1.0
Gross Unrealized Losses	0.1		1.6
Fair value	60.0		55.6
Fixed Maturities [Member] | Obligations Of States And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	560.7		589.3
d losses and fair 31, 2011 were as Gross Unrealized Gains	49.4		47.4
Gross Unrealized Losses	0.3		0.3
Fair value	609.8		636.4
Fixed Maturities [Member] | Credit-Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	366.4		378.3
d losses and fair 31, 2011 were as Gross Unrealized Gains	24.6		10.4
Gross Unrealized Losses	0.7		5.2
Fair value	390.3		383.5
Fixed Maturities [Member] | Credit-Industrial [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	411.2		411.6
d losses and fair 31, 2011 were as Gross Unrealized Gains	32.1		20.8
Gross Unrealized Losses	1.0		3.4
Fair value	442.3		429.0
Fixed Maturities [Member] | Credit-Utility [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	183.0		163.9
d losses and fair 31, 2011 were as Gross Unrealized Gains	12.3		7.5
Gross Unrealized Losses	0.6		1.2
Fair value	194.7		170.2
Fixed Maturities [Member] | CMO/MBS-Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	418.7	[2]	516.0	[2]
d losses and fair 31, 2011 were as Gross Unrealized Gains	25.9	[2]	30.7	[2]
Gross Unrealized Losses	0.1	[2]	0.2	[2]
Fair value	444.5	[2]	546.5	[2]
Fixed Maturities [Member] | CMO/MBS-Non Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13.6		18.5
d losses and fair 31, 2011 were as Gross Unrealized Gains	0.8		0.7
Gross Unrealized Losses	0.2		0.8
Fair value	14.2		18.4
Fixed Maturities [Member] | CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104.9	[3]	100.7	[3]
d losses and fair 31, 2011 were as Gross Unrealized Gains	6.3	[3]	5.2	[3]
Gross Unrealized Losses	0.2	[3]	0.3	[3]
Fair value	111.0	[3]	105.6	[3]
Fixed Maturities [Member] | ABS-Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11.1	[4]	15.3	[4]
d losses and fair 31, 2011 were as Gross Unrealized Gains	0.3	[4]	0.1	[4]
Gross Unrealized Losses	1.0	[4]	2.0	[4]
Fair value	10.4	[4]	13.4	[4]
Fixed Maturities [Member] | ABS-Non Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77.1		47.0
d losses and fair 31, 2011 were as Gross Unrealized Gains	1.2		1.0
Gross Unrealized Losses	0		0
Fair value	78.3		48.0
Fixed Maturities [Member] | Foreign Denominated, Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	250.3		224.7
d losses and fair 31, 2011 were as Gross Unrealized Gains	10.5		9.1
Gross Unrealized Losses	5.3		8.8
Fair value	255.5		225.0
Fixed Maturities [Member] | Foreign Denominated, Credit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125.9		91.1
d losses and fair 31, 2011 were as Gross Unrealized Gains	6.4		2.2
Gross Unrealized Losses	2.7		4.7
Fair value	$ 129.6		$ 88.6

[1] (1) Includes corporate bonds backed by the Federal Deposit Insurance Corporation of $4.3 million amortized cost, $4.3 million fair value.

[2] Collateralized mortgage obligations/mortgage-backed securities ("CMO/MBS").

[3] Commercial mortgage-backed securities ("CMBS").

[4] Asset-backed securities ("ABS").