Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) (USD $)					12 Months Ended		12 Months Ended																12 Months Ended																		12 Months Ended						12 Months Ended				12 Months Ended				12 Months Ended									12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2012 Fair value measured on a recurring basis Discontinued Operations	Dec. 31, 2011 Fixed maturity investments	Dec. 31, 2012 Level 1 Inputs	Dec. 31, 2011 Level 1 Inputs	Dec. 31, 2012 Level 1 Inputs Fair value measured on a recurring basis	Dec. 31, 2011 Level 1 Inputs Fair value measured on a recurring basis	Dec. 31, 2012 Level 1 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 1 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2012 Level 1 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2011 Level 1 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2012 Level 1 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2011 Level 1 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2012 Level 1 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 1 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2012 Level 2 Inputs	Dec. 31, 2011 Level 2 Inputs	Dec. 31, 2012 Level 2 Inputs Fair value measured on a recurring basis	Dec. 31, 2011 Level 2 Inputs Fair value measured on a recurring basis	Dec. 31, 2012 Level 2 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 2 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2012 Level 2 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2011 Level 2 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2012 Level 2 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2011 Level 2 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2012 Level 2 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 2 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2012 Level 3 Inputs Fair value measured on a recurring basis	Dec. 31, 2011 Level 3 Inputs Fair value measured on a recurring basis	Dec. 31, 2012 Level 3 Inputs Fixed maturity investments Investment	Dec. 31, 2011 Level 3 Inputs Fixed maturity investments Investment	Dec. 31, 2012 Level 3 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 3 Inputs Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2012 Level 3 Inputs Common equity securities	Dec. 31, 2011 Level 3 Inputs Common equity securities	Dec. 31, 2012 Level 3 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2011 Level 3 Inputs Common equity securities Fair value measured on a recurring basis	Dec. 31, 2012 Level 3 Inputs Convertible fixed maturity investments	Dec. 31, 2011 Level 3 Inputs Convertible fixed maturity investments	Dec. 31, 2012 Level 3 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2011 Level 3 Inputs Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2012 Level 3 Inputs Other long-term investments		Dec. 31, 2011 Level 3 Inputs Other long-term investments		Dec. 31, 2012 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2011 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2010 Level 3 Inputs Other long-term investments Fair value measured on a recurring basis	Dec. 31, 2012 Fair value		Dec. 31, 2011 Fair value		Dec. 31, 2012 Fair value Fair value measured on a recurring basis	Dec. 31, 2011 Fair value Fair value measured on a recurring basis	Dec. 31, 2012 Fair value Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2011 Fair value Fixed maturity investments Fair value measured on a recurring basis		Dec. 31, 2012 Fair value Common equity securities Fair value measured on a recurring basis	Dec. 31, 2011 Fair value Common equity securities Fair value measured on a recurring basis	Dec. 31, 2012 Fair value Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2011 Fair value Convertible fixed maturity investments Fair value measured on a recurring basis	Dec. 31, 2012 Fair value Other long-term investments Fair value measured on a recurring basis		Dec. 31, 2011 Fair value Other long-term investments Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments							$ 1,879,100,000	$ 1,894,400,000	$ 1,985,700,000	$ 1,879,100,000	$ 422,200,000	[1]	$ 365,700,000	[2]	$ 932,900,000	$ 667,400,000	$ 0	$ 0	$ 0	[3]	$ 0	[4]	$ 6,088,200,000	$ 5,477,400,000	$ 5,206,100,000	$ 6,088,200,000	$ 5,019,200,000	[1]	$ 5,889,100,000	[2]	$ 59,500,000	$ 55,300,000	$ 127,400,000	$ 143,800,000	$ 0	[3]	$ 0	[4]	$ 389,500,000	$ 379,500,000	$ 78,900,000	$ 128,400,000	$ 92,900,000	[1]	$ 78,900,000	[2]	$ 32,300,000	$ 71,200,000	$ 37,300,000	$ 32,300,000	$ 0	$ 0.0	$ 0	$ 0	$ 268,300,000	[5]	$ 330,200,000	[5]	$ 259,300,000	[3]	$ 268,300,000	[4]		$ 8,346,800,000	[5],[6],[7],[8]	$ 7,901,600,000	[5]	$ 7,581,300,000	$ 8,346,800,000	$ 5,534,300,000	[1]	$ 6,333,700,000	[2]	$ 1,029,700,000	$ 755,000,000	$ 127,400,000	$ 143,800,000	$ 259,300,000	[3]	$ 268,300,000	[4]
Total realized and unrealized gains (losses)							46,800,000	(1,400,000)															53,600,000	113,200,000																	8,700,000	(8,100,000)					12,400,000	(4,700,000)			0	0			(3,300,000)		19,500,000							118,200,000		118,500,000
Foreign currency gains (losses) through OCI							8,900,000	4,600,000															81,900,000	(76,100,000)																	800,000	(4,400,000)					200,000	1,600,000			0	0			3,700,000		(5,000,000)							95,500,000		(79,300,000)
Amortization/Accretion							(800,000)	2,300,000															(48,000,000)	(54,200,000)																	(800,000)	300,000					0	0			0	0			0		0							(49,600,000)		(51,600,000)
Purchases							7,266,500,000	10,653,600,000															4,927,100,000	8,905,600,000																	144,400,000	213,700,000					3,100,000	19,700,000			0	0			39,400,000		58,400,000							12,380,500,000		19,851,000,000
Sales							(7,214,800,000)	(10,674,400,000)															(5,937,000,000)	(8,528,700,000)																	(99,400,000)						(10,100,000)	(55,500,000)			0	0			(48,800,000)		(134,800,000)							(13,310,100,000)		(19,393,400,000)
Transfers in							0	0															62,400,000	269,200,000																	22,100,000	18,200,000					0	0			0	0			0		0							84,500,000		287,400,000
Transfers out							0	0															(22,100,000)	(18,200,000)																	(61,800,000)	(269,200,000)					(600,000)	0			0	0			0		0							(84,500,000)		(287,400,000)
Fair value investments							1,985,700,000	1,879,100,000	1,985,700,000	1,879,100,000	422,200,000	[1]	365,700,000	[2]	932,900,000	667,400,000	0	0	0	[3]	0	[4]	5,206,100,000	6,088,200,000	5,206,100,000	6,088,200,000	5,019,200,000	[1]	5,889,100,000	[2]	59,500,000	55,300,000	127,400,000	143,800,000	0	[3]	0	[4]	389,500,000	379,500,000	92,900,000	78,900,000	92,900,000	[1]	78,900,000	[2]	37,300,000	32,300,000	37,300,000	32,300,000	0	0	0	0	259,300,000		268,300,000	[5]	259,300,000	[3]	268,300,000	[4]		7,581,300,000	[7],[8]	8,346,800,000	[5],[6],[7],[8]	7,581,300,000	8,346,800,000	5,534,300,000	[1]	6,333,700,000	[2]	1,029,700,000	755,000,000	127,400,000	143,800,000	259,300,000	[3]	268,300,000	[4]
Carrying value of investment accounted for using the equity method																																																											35,000,000		33,000,000		41,900,000
Change in fair value of derivatives, included in other revenues					(100,000)
Fixed maturity investments reclassified to assets held for sale (1)	(338,100,000)	[9]	(111,800,000)	[9]
Number of investments																																									2	10
Securities of level 3 measurements recategorized as level 2 measurements																							61,800,000	269,200,000																	22,100,000
Difference in value between securities determined using the industry standard pricing models and securities based upon quoted prices provided by a third party						$ 1,600,000

[1] Carrying value includes $338.1 that is classified as assets held for sale relating to discontinued operations.

[2] Carrying value includes $111.8 that is classified as assets held for sale relating to discontinued operations.

[3] Excludes carrying value of $35.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.

[4] Excludes carrying value of $33.0 associated with other long-term investments accounted for using the equity method.

[5] Excludes carrying value of $33.0 and $41.9 at December 31, 2011 and January 1, 2011 associated with other long-term investment limited partnerships accounted for using the equity method.

[6] Carrying value includes $111.8 that is classified as assets held for sale relating to AutoOne discontinued operations.

[7] Excludes carrying value of $33.0 and $35.0 at January 1, 2012 and December 31, 2012 associated with other long-term investments accounted for using the equity method and $(0.1) at December 31, 2012 related to currency forward contracts.

[8] Carrying value includes $111.8 and $338.1 at January 1, 2012 and December 31, 2012 that is classified as assets held for sale relating to discontinued operations.

[9] Assets held for sale related to discontinued operations. See Note 20.