(Rollforward of Fair Value Measurements by Level) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015	$ 5,991,400,000	$ 6,766,500,000	$ 6,766,500,000
Total realized and unrealized gains (losses)	101,400,000	64,400,000
Foreign currency gains (losses) through OCI	(129,600,000)	(13,800,000)
Amortization/Accretion	(10,100,000)	(11,300,000)
Purchases	1,348,500,000	1,225,600,000
Sales	(1,344,400,000)	(1,299,300,000)
Net change in investments related to (sales) purchases of consolidated affiliates		4,600,000
Transfers in	5,600,000	0
Transfers out	(5,600,000)	0
Balance at March 31, 2015	5,957,200,000	6,736,700,000	5,991,400,000
Other Investments	402,600,000		407,000,000
Realized and unrealized gain	77,100,000	48,100,000
Net realized and unrealized investment gains	103,400,000	63,800,000	63,800,000
Short-term investments, at amortized cost (which approximates fair value)	929,200,000		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
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	1,376,700,000
Total realized and unrealized gains (losses)	18,300,000	22,200,000
Foreign currency gains (losses) through OCI	(9,600,000)	(2,900,000)
Amortization/Accretion	(100,000)	(200,000)
Purchases	222,900,000	352,600,000
Sales	(269,800,000)	(180,100,000)
Net change in investments related to (sales) purchases of consolidated affiliates		(2,700,000)
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	805,000,000	1,565,600,000
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	4,982,200,000
Total realized and unrealized gains (losses)	87,300,000	34,700,000
Foreign currency gains (losses) through OCI	(115,600,000)	(10,500,000)
Amortization/Accretion	(10,000,000)	(11,100,000)
Purchases	1,047,700,000	840,400,000
Sales	(1,059,000,000)	(1,115,500,000)
Net change in investments related to (sales) purchases of consolidated affiliates		7,300,000
Transfers in	5,600,000	0
Transfers out	0	0
Balance at March 31, 2015	4,594,000,000	4,727,500,000
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	93,000,000
Total realized and unrealized gains (losses)	300,000	400,000
Foreign currency gains (losses) through OCI	(900,000)	(100,000)
Amortization/Accretion	0	0
Purchases	68,600,000	19,600,000
Sales	0	0
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	(5,600,000)	0
Balance at March 31, 2015	139,100,000	112,900,000
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	46,100,000
Total realized and unrealized gains (losses)	1,800,000	800,000
Foreign currency gains (losses) through OCI	0	0
Amortization/Accretion	0	0
Purchases	0	300,000
Sales	(9,400,000)	0
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	32,600,000	47,200,000
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	6,100,000
Total realized and unrealized gains (losses)	(4,300,000)	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 (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	3,900,000	6,100,000
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	262,400,000
Total realized and unrealized gains (losses)	(2,000,000)	6,300,000
Foreign currency gains (losses) through OCI	(3,500,000)	(300,000)
Amortization/Accretion	0	0
Purchases	9,300,000	12,700,000
Sales	(6,200,000)	(3,700,000)
Net change in investments related to (sales) purchases of consolidated affiliates		0
Transfers in	0	0
Transfers out	0	0
Balance at March 31, 2015	382,600,000	277,400,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	6,886,400,000		6,863,100,000
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	1,731,900,000		1,712,100,000
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	4,596,300,000		4,640,900,000
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	558,200,000		510,100,000
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	4,771,900,000		4,784,300,000
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 March 31, 2015	129,300,000		155,400,000
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 March 31, 2015	4,503,500,000		4,552,200,000
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 March 31, 2015	139,100,000		76,700,000
Fair value measured on a recurring basis | Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	15,800,000		20,500,000
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 March 31, 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 March 31, 2015	11,900,000		12,300,000
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 March 31, 2015	3,900,000		8,200,000
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	382,600,000		385,000,000
Carrying value of investment accounted for using the equity method	3,800,000
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 March 31, 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 March 31, 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 March 31, 2015	382,600,000		385,000,000
Carrying value of investment accounted for using the equity method		5,700,000	5,200,000	6,800,000
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at March 31, 2015	929,200,000		871,700,000
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 March 31, 2015	926,800,000		868,800,000
Short-term investments, at amortized cost (which approximates fair value)	929,200,000	674,000,000	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 March 31, 2015	2,400,000		2,900,000
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 March 31, 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		222,200,000		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 gains	1,000,000		700,000
Forward Contracts | Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Carrying value of investment accounted for using the equity method		(100,000)		(100,000)
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16,200,000	18,500,000	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	16,200,000		16,800,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment gains	$ 3.5