Investment Securities (Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	$ 6,950.7	$ 6,766.5	$ 6,950.7
Total realized and unrealized gains (losses)		286.5	182.0
Foreign currency gains (losses) through OCI		(253.6)	11.6
Amortization/Accretion		(44.1)	52.2
Purchases		5,602.1	4,635.5
Sales		(6,387.5)	(4,979.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		21.5
Fair Value, Assets Measured on Recurring Basis, Exchange		0
Transfers in		97.8	217.6
Transfers out		(97.8)	(217.6)
Fair value investments		5,991.4	6,766.5	6,950.7
Maximum price difference provided by pricing services	1.3	1.0
Net realized and unrealized investment gains		283.9	161.7	118.2
Discontinued Operations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fixed maturity investments reclassified to assets held for sale (1)			(236.3)	(338.1)
Fair value measured on a recurring basis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments		6,863.1	7,402.4
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		4,784.3	5,266.8
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		20.5	80.5
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Carrying value of investment accounted for using the equity method		23.2
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		385.0	262.4
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					630.6
Fair value investments		871.7	635.9		630.6
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	1,355.1	1,376.7	1,355.1
Total realized and unrealized gains (losses)		88.1	221.9
Foreign currency gains (losses) through OCI		(24.8)	(0.3)
Amortization/Accretion		(0.6)	(1.0)
Purchases		1,599.2	862.1
Sales		(2,197.1)	(1,078.9)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		(2.7)
Fair Value, Assets Measured on Recurring Basis, Exchange		4.7
Transfers in		0	1.8
Transfers out		(0.2)	0
Fair value investments		843.3	1,376.7
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		1,712.1	1,998.2
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		155.4	340.3
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	0
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		868.8	621.5
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	5,206.1	4,982.2	5,206.1
Total realized and unrealized gains (losses)		163.4	(56.9)
Foreign currency gains (losses) through OCI		(222.8)	12.5
Amortization/Accretion		(43.6)	(51.2)
Purchases		3,772.4	3,689.6
Sales		(4,122.6)	(3,842.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		24.2
Fair Value, Assets Measured on Recurring Basis, Exchange		(13.0)
Transfers in		97.8	119.4
Transfers out		0	(97.2)
Fair value investments		4,638.0	4,982.2
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		4,640.9	4,996.6
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		4,552.2	4,833.5
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		12.3	74.4
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	0
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			(0.1)
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		2.9	14.4
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		510.1	407.6
Level 3 Inputs | Fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	92.9	93.0	92.9
Total realized and unrealized gains (losses)		2.2	(2.7)
Foreign currency gains (losses) through OCI		(1.0)	0.3
Amortization/Accretion		0.1	0
Purchases		71.7	37.9
Sales		0	(6.3)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Fair Value, Assets Measured on Recurring Basis, Exchange		8.3
Transfers in	90.3	0	90.3
Transfers out		(97.6)	(119.4)
Fair value investments		76.7	93.0
Number of investments		6	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		76.7	93.0
Level 3 Inputs | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	37.3	46.1	37.3
Total realized and unrealized gains (losses)		6.2	1.0
Foreign currency gains (losses) through OCI		0	0
Amortization/Accretion		0	0
Purchases		5.0	8.8
Sales		(17.1)	0
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Fair Value, Assets Measured on Recurring Basis, Exchange		0
Transfers in		0	0
Transfers out		0	(1.0)
Fair value investments		40.2	46.1
Level 3 Inputs | Convertible fixed maturity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	0	6.1	0
Total realized and unrealized gains (losses)		0.6	0
Foreign currency gains (losses) through OCI		0	0
Amortization/Accretion		0	0
Purchases		1.5	0
Sales		0	0
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Fair Value, Assets Measured on Recurring Basis, Exchange		0
Transfers in		0	6.1
Transfers out		0	0
Fair value investments		8.2	6.1
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		8.2	6.1
Level 3 Inputs | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value investments	259.3	262.4	259.3
Total realized and unrealized gains (losses)		26.0	18.7
Foreign currency gains (losses) through OCI		(5.0)	(0.9)
Amortization/Accretion		0	0
Purchases		152.3	37.1
Sales		(50.7)	(51.8)
Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates		0
Fair Value, Assets Measured on Recurring Basis, Exchange		0
Transfers in		0	0
Transfers out		0	0
Fair value investments		385.0	262.4
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		385.0	262.4
Carrying value of investment accounted for using the equity method		23.2	26.6	35.0
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
Prospector
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains		4.3	20.6
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net realized and unrealized investment gains		$ 2.8	$ 0.2