Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Investment securities
Net realized and unrealized investment gains	$ 161,700,000		$ 118,200,000		$ 74,100,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Sales	(4,924,800,000)		(6,997,500,000)		(5,034,000,000)
Discontinued Operations
Investment securities
Net realized and unrealized investment gains	0		0		700,000
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments reclassified to assets held for sale (1)	(236,300,000)		(338,100,000)		(111,800,000)
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,355,100,000		1,033,100,000
Total realized and unrealized gains (losses)	221,900,000		52,000,000
Foreign currency gains (losses) through OCI	(300,000)		4,400,000
Amortization/Accretion	(1,000,000)		(800,000)
Purchases	862,100,000		1,216,900,000
Sales	(1,078,900,000)		(950,500,000)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	16,000,000
Transfers in	1,800,000		0
Transfers out	0		0
Fair value investments	1,376,700,000		1,355,100,000
Level 1 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,998,200,000		1,985,700,000
Level 1 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	340,300,000	[1]	422,200,000	[2]
Level 1 Inputs | Convertible fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0		0
Level 1 Inputs | 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	0	[3]	0	[4]
Level 1 Inputs | Short-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	621,500,000		630,600,000
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,206,100,000		6,088,200,000
Total realized and unrealized gains (losses)	(56,900,000)		53,600,000
Foreign currency gains (losses) through OCI	12,500,000		81,900,000
Amortization/Accretion	(51,200,000)		(48,000,000)
Purchases	3,689,600,000		4,927,100,000
Sales	(3,842,800,000)		(5,937,000,000)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	2,700,000
Transfers in	119,400,000		62,400,000
Transfers out	(97,200,000)		(22,100,000)
Fair value investments	4,982,200,000		5,206,100,000
Level 2 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,996,600,000		5,206,100,000
Level 2 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	4,833,500,000	[1]	5,019,200,000	[2]
Level 2 Inputs | Convertible fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	74,400,000		127,400,000
Level 2 Inputs | 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	0	[3]	0	[4]
Level 2 Inputs | Forward contracts | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Change in fair value of derivatives, included in other revenue	(100,000)		0
Level 2 Inputs | Short-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	14,400,000		0
Level 3 Inputs | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	407,600,000		389,500,000
Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	92,900,000		78,900,000
Total realized and unrealized gains (losses)	(2,700,000)		8,700,000
Foreign currency gains (losses) through OCI	300,000		800,000
Amortization/Accretion	0		(800,000)
Purchases	37,900,000		144,400,000
Sales	(6,300,000)		(99,400,000)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Transfers in	90,300,000		22,100,000
Transfers out	(119,400,000)		(61,800,000)
Fair value investments	93,000,000		92,900,000
Number of investments	3
Level 3 Inputs | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	93,000,000	[1]	92,900,000	[2]
Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	37,300,000		32,300,000
Total realized and unrealized gains (losses)	1,000,000		12,400,000
Foreign currency gains (losses) through OCI	0		200,000
Amortization/Accretion	0		0
Purchases	8,800,000		3,100,000
Sales	0		(10,100,000)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Transfers in	0		0
Transfers out	(1,000,000)		(600,000)
Fair value investments	46,100,000		37,300,000
Level 3 Inputs | Convertible fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0		0.0
Total realized and unrealized gains (losses)	0		0
Foreign currency gains (losses) through OCI	0		0
Amortization/Accretion	0		0
Purchases	0		0
Sales	0		0
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Transfers in	6,100,000		0
Transfers out	0		0
Fair value investments	6,100,000		0
Level 3 Inputs | Convertible fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	6,100,000		0
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259,300,000	[5],[6]	268,300,000	[5]
Total realized and unrealized gains (losses)	18,700,000		(3,300,000)
Foreign currency gains (losses) through OCI	(900,000)		3,700,000
Amortization/Accretion	0		0
Purchases	37,100,000		39,400,000
Sales	(51,800,000)		(48,800,000)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	0
Transfers in	0		0
Transfers out	0		0
Fair value investments	262,400,000	[6]	259,300,000	[5],[6]
Level 3 Inputs | 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	262,400,000	[3]	259,300,000	[4]
Carrying value of investment accounted for using the equity method	26,600,000		35,000,000		33,000,000
Level 3 Inputs | Short-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	0		0
Fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	6,950,700,000	[10],[5],[6],[7],[8],[9]	7,500,800,000	[5],[7],[8]
Total realized and unrealized gains (losses)	182,000,000	[11]	123,400,000
Foreign currency gains (losses) through OCI	11,600,000		91,000,000
Amortization/Accretion	(52,200,000)		49,600,000
Purchases	4,635,500,000		6,330,900,000
Sales	(4,979,800,000)		7,045,800,000
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates	18,700,000
Transfers in	217,600,000		84,500,000
Transfers out	(217,600,000)		84,500,000
Fair value investments	6,766,500,000	[10],[6],[9]	6,950,700,000	[10],[5],[6],[7],[8],[9]
Fair value | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	7,402,400,000		7,581,300,000
Fair value | Fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,266,800,000	[1]	5,534,300,000	[2]
Fair value | Convertible fixed maturity investments | Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	80,500,000		127,400,000
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	262,400,000	[3]	259,300,000	[4]
Fair value | Short-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	635,900,000		630,600,000		0
Prospector
Investment securities
Net realized and unrealized investment gains	0
Less than market price [Member] | Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers in	$ 1,300,000

[1] Carrying value includes $236.3 that is classified as assets held for sale relating to discontinued operations.

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

[3] Excludes carrying value of $26.6 associated with other long-term investment limited partnerships accounted for using the equity method and $(.1) related to currency forward contracts.

[4] Excludes carrying value of $35.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(.1) related to currency forward contracts.

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

[6] Excludes carrying value of $26.6 and $35.0 at December 31, 2013 and January 1, 2013 associated with other long-term investments accounted for using the equity method and $(.1) at December 31, 2013 related to currency forward contracts.

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

[8] Excludes carrying value of $630.6 and $846.0 at December 31, 2012 and January 1, 2012 classified as short-term investments.

[9] Excludes carrying value of $635.9 and $630.6 at December 31, 2013 and January 1, 2013 classified as short-term investments.

[10] Carrying value includes $236.3 and $338.1 at December 31, 2013 and January 1, 2013 that is classified as assets held for sale relating to discontinued operations.

[11] Excludes $20.6 of realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities and $.2 of realized and unrealized gains associated with short-term investments.