Rollforward of Fair Value Measurements by Level (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			$ 5,991,400,000	$ 6,766,500,000
Total realized and unrealized gains (losses)			34,300,000	180,700,000
Foreign currency gains (losses) through OCI			(70,200,000)	(59,400,000)
Amortization/Accretion			(21,200,000)	(22,400,000)
Purchases			3,057,400,000	2,685,500,000
Sales			(2,960,800,000)	(2,778,500,000)
Net change in investments related to (sales) purchases of consolidated affiliates				4,600,000
Fair Value, Assets Measured on Recurring Basis, Other			(67,500,000)
Transfers in			73,900,000	28,100,000
Transfers out			(73,900,000)	(28,100,000)
Balance at June 30, 2015	$ 5,963,400,000	$ 6,777,000,000	5,963,400,000	6,777,000,000
Other Investments	390,100,000		390,100,000		$ 407,000,000
Realized and unrealized gain	(44,600,000)	90,900,000	32,500,000	139,000,000
Net realized and unrealized investment (losses) gains	(61,100,000)	113,900,000	42,300,000	177,700,000
Short-term investments, at amortized cost (which approximates fair value)	840,500,000		$ 840,500,000		871,700,000
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1,000,000
Price Difference Between the Investment Management Pricing and Third Party Pricing	1,000,000		1,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			843,300,000	1,376,700,000
Total realized and unrealized gains (losses)			13,100,000	67,300,000
Foreign currency gains (losses) through OCI			(5,400,000)	(6,800,000)
Amortization/Accretion			(100,000)	(300,000)
Purchases			945,900,000	953,500,000
Sales			(1,017,800,000)	(896,100,000)
Net change in investments related to (sales) purchases of consolidated affiliates				(2,700,000)
Fair Value, Assets Measured on Recurring Basis, Other			(49,100,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	729,900,000	1,491,600,000	729,900,000	1,491,600,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			4,638,000,000	4,982,200,000
Total realized and unrealized gains (losses)			31,300,000	89,800,000
Foreign currency gains (losses) through OCI			(62,300,000)	(50,800,000)
Amortization/Accretion			(21,100,000)	(22,000,000)
Purchases			2,002,400,000	1,632,000,000
Sales			(1,915,500,000)	(1,875,600,000)
Net change in investments related to (sales) purchases of consolidated affiliates				7,300,000
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			73,900,000	19,600,000
Transfers out			0	(8,500,000)
Balance at June 30, 2015	4,746,700,000	4,774,000,000	4,746,700,000	4,774,000,000
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			76,700,000	93,000,000
Total realized and unrealized gains (losses)			0	900,000
Foreign currency gains (losses) through OCI			(200,000)	(400,000)
Amortization/Accretion			0	(100,000)
Purchases			76,000,000	76,100,000
Sales			0	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			0	8,500,000
Transfers out			(73,900,000)	(19,600,000)
Balance at June 30, 2015	$ 78,600,000	158,400,000	$ 78,600,000	158,400,000
Number of Investments	5		5
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			$ 40,200,000	46,100,000
Total realized and unrealized gains (losses)			6,900,000	2,700,000
Foreign currency gains (losses) through OCI			0	0
Amortization/Accretion			0	0
Purchases			0	5,000,000
Sales			(9,500,000)	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			(4,500,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	$ 33,100,000	53,800,000	33,100,000	53,800,000
Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			8,200,000	6,100,000
Total realized and unrealized gains (losses)			(3,700,000)	3,200,000
Foreign currency gains (losses) through OCI			0	0
Amortization/Accretion			0	0
Purchases			0	1,500,000
Sales			0	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	4,500,000	10,800,000	4,500,000	10,800,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			385,000,000	262,400,000
Total realized and unrealized gains (losses)			(13,300,000)	16,800,000
Foreign currency gains (losses) through OCI			(2,300,000)	(1,400,000)
Amortization/Accretion			0	0
Purchases			33,100,000	17,400,000
Sales			(18,000,000)	(6,800,000)
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			(13,900,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	370,600,000	288,400,000	370,600,000	288,400,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			6,863,100,000
Balance at June 30, 2015	6,803,900,000		6,803,900,000
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			1,712,100,000
Balance at June 30, 2015	1,568,200,000		1,568,200,000
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			4,640,900,000
Balance at June 30, 2015	4,748,900,000		4,748,900,000
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			510,100,000
Balance at June 30, 2015	486,800,000		486,800,000
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			4,784,300,000
Balance at June 30, 2015	4,874,500,000		4,874,500,000
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			155,400,000
Balance at June 30, 2015	150,300,000		150,300,000
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			4,552,200,000
Balance at June 30, 2015	4,645,600,000		4,645,600,000
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			76,700,000
Balance at June 30, 2015	78,600,000		78,600,000
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			20,500,000
Balance at June 30, 2015	4,500,000		4,500,000
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
Balance at June 30, 2015	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			12,300,000
Balance at June 30, 2015	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			8,200,000
Balance at June 30, 2015	4,500,000		4,500,000
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			385,000,000
Balance at June 30, 2015	370,600,000		370,600,000
Carrying value of investment accounted for using the equity method	3,900,000		3,900,000
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
Balance at June 30, 2015	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
Balance at June 30, 2015	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			385,000,000
Balance at June 30, 2015	370,600,000		370,600,000
Carrying value of investment accounted for using the equity method		5,600,000		5,600,000	5,200,000	$ 6,800,000
Fair value measured on a recurring basis | Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at June 30, 2015	(200,000)		(200,000)
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			871,700,000
Balance at June 30, 2015	840,500,000		840,500,000
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			868,800,000
Balance at June 30, 2015	838,300,000		838,300,000
Short-term investments, at amortized cost (which approximates fair value)	840,500,000	564,000,000	840,500,000	564,000,000	871,700,000	635,900,000
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			2,900,000
Balance at June 30, 2015	2,200,000		2,200,000
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
Balance at June 30, 2015	0		0
Segment, Discontinued Operations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale		203,800,000		203,800,000		236,300,000
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			900,000	1,800,000
Forward Contracts | 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				(100,000)
Balance at June 30, 2015	(200,000)	100,000	(200,000)	100,000
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15,800,000	$ 18,000,000	15,800,000	$ 18,000,000	16,800,000	$ 19,100,000
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	$ 15,800,000		15,800,000		$ 16,800,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			$ 9.3