Rollforward of Fair Value Measurements by Level (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			$ 3,343.6	$ 3,889.0
Total realized and unrealized (losses) gains			(32.7)	49.7
Amortization/Accretion			(15.2)	(14.3)
Purchases			1,672.0	2,016.2
Sales			(1,540.9)	(2,340.7)
Net change in investments related to (sales) purchases of consolidated affiliates				10.8
Fair Value, Assets Measured on Recurring Basis, Other			(61.4)
Transfers in			41.7	42.1
Transfers out			(41.7)	(42.1)
Balance at September 30, 2015	$ 3,365.4	$ 3,610.7	3,365.4	3,610.7
Other Investments	294.6		294.6		$ 318.0
Realized and unrealized gain	(34.5)	(21.9)	(27.0)	52.2
Net realized and unrealized investment (losses) gains	(43.9)	(27.5)	(33.9)	64.4
Short-term investments, at amortized cost (which approximates fair value)	325.0		$ 325.0		376.8
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1.0
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			550.6	997.2
Total realized and unrealized (losses) gains			(19.5)	30.7
Amortization/Accretion			0.0	0.1
Purchases			550.0	661.2
Sales			(618.2)	(926.2)
Net change in investments related to (sales) purchases of consolidated affiliates				3.4
Fair Value, Assets Measured on Recurring Basis, Other			(43.5)
Transfers in			0.0	0.0
Transfers out			(1.2)	(0.2)
Balance at September 30, 2015	418.2	766.2	418.2	766.2
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			2,372.9	2,575.4
Total realized and unrealized (losses) gains			0.6	12.5
Amortization/Accretion			(15.2)	(14.3)
Purchases			1,056.0	1,265.3
Sales			(894.4)	(1,392.5)
Net change in investments related to (sales) purchases of consolidated affiliates				7.4
Fair Value, Assets Measured on Recurring Basis, Other			0.0
Transfers in			41.7	42.1
Transfers out			0.0	0.0
Balance at September 30, 2015	2,561.6	2,495.9	2,561.6	2,495.9
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			76.4	81.8
Total realized and unrealized (losses) gains			(0.9)	2.7
Amortization/Accretion			0.0	(0.1)
Purchases			35.3	45.5
Sales			0.0	0.0
Net change in investments related to (sales) purchases of consolidated affiliates				0.0
Fair Value, Assets Measured on Recurring Basis, Other			0.0
Transfers in			0.0	0.0
Transfers out			(40.5)	(41.9)
Balance at September 30, 2015	70.3	$ 88.0	70.3	$ 88.0
Number of Investments		6		6
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			39.5	$ 45.4
Total realized and unrealized (losses) gains			7.7	3.7
Amortization/Accretion			0.0	0.0
Purchases			0.0	0.0
Sales			(9.4)	(10.9)
Net change in investments related to (sales) purchases of consolidated affiliates				0.0
Fair Value, Assets Measured on Recurring Basis, Other			(3.6)
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2015	34.2	$ 38.2	34.2	38.2
Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			8.2	6.1
Total realized and unrealized (losses) gains			(2.6)	3.3
Amortization/Accretion			0.0	0.0
Purchases			0.0	1.5
Sales			0.0	0.0
Net change in investments related to (sales) purchases of consolidated affiliates				0.0
Fair Value, Assets Measured on Recurring Basis, Other			0.0
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2015	5.6	10.9	5.6	10.9
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			296.0	183.1
Total realized and unrealized (losses) gains			(18.0)	(3.2)
Amortization/Accretion			0.0	0.0
Purchases			30.7	42.7
Sales			(18.9)	(11.1)
Net change in investments related to (sales) purchases of consolidated affiliates				0.0
Fair Value, Assets Measured on Recurring Basis, Other			(14.3)
Transfers in			0.0	0.0
Transfers out			0.0	0.0
Balance at September 30, 2015	275.5	211.5	275.5	211.5
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			3,720.4
Balance at September 30, 2015	3,690.4		3,690.4
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			926.9
Balance at September 30, 2015	743.2		743.2
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			2,373.4
Balance at September 30, 2015	2,561.6		2,561.6
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			420.1
Balance at September 30, 2015	385.6		385.6
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			2,422.0
Balance at September 30, 2015	2,602.2		2,602.2
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			51.8
Balance at September 30, 2015	62.4		62.4
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			2,293.8
Balance at September 30, 2015	2,469.5		2,469.5
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			76.4
Balance at September 30, 2015	70.3		70.3
Fair value measured on a recurring basis | Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			13.9
Balance at September 30, 2015	5.6		5.6
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0.0
Balance at September 30, 2015	0.0		0.0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			5.7
Balance at September 30, 2015	0.0		0.0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			8.2
Balance at September 30, 2015	5.6		5.6
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			296.0
Balance at September 30, 2015	275.5		275.5
Carrying value of investment accounted for using the equity method	3.9		3.9
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0.0
Balance at September 30, 2015	0.0		0.0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0.0
Balance at September 30, 2015	0.0		0.0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			296.0
Balance at September 30, 2015	275.5		275.5
Carrying value of investment accounted for using the equity method		5.1		5.1	5.2	$ 6.8
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			376.8
Balance at September 30, 2015	325.0		325.0
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			376.3
Balance at September 30, 2015	325.0		325.0
Short-term investments, at amortized cost (which approximates fair value)	325.0	402.7	325.0	402.7	376.8	310.4
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0.5
Balance at September 30, 2015	0.0		0.0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0.0
Balance at September 30, 2015	0.0		0.0
Discontinued Operations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains	(0.9)	48.3	31.4	134.1
Fixed maturity investments reclassified to assets held for sale		203.9		203.9		236.3
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			0.8	0.5
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15.2	$ 17.3	15.2	$ 17.3	16.8	$ 19.1
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 15.2		$ 15.2		$ 16.8