(Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			6 Months Ended																																																12 Months Ended	6 Months Ended																6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Level 1 Inputs	Jun. 30, 2013 Level 1 Inputs	Jun. 30, 2014 Level 2 Inputs	Jun. 30, 2013 Level 2 Inputs	Jun. 30, 2014 Fair Value	Jun. 30, 2013 Fair Value	Jun. 30, 2014 Fixed maturity investments. Level 3 Inputs	Jun. 30, 2013 Fixed maturity investments. Level 3 Inputs	Jun. 30, 2014 Common equity securities Level 3 Inputs	Jun. 30, 2013 Common equity securities Level 3 Inputs	Jun. 30, 2014 Convertible Debt Securities Level 3 Inputs	Jun. 30, 2013 Convertible Debt Securities Level 3 Inputs	Jun. 30, 2014 Other long-term investments Level 3 Inputs	Jun. 30, 2013 Other long-term investments Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Level 1 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Level 2 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Level 3 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Fair Value	Jun. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Fair Value	Jun. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Fair Value	Jun. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Jun. 30, 2013 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Other long-term investments Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Forward Contracts Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs	Jun. 30, 2013 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs	Jun. 30, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 2 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 2 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 3 Inputs	Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 3 Inputs	Jun. 30, 2014 Fair value measured on a recurring basis Short-term investments Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Fair Value	Jun. 30, 2014 Segment, Discontinued Operations	Dec. 31, 2013 Segment, Discontinued Operations	Jun. 30, 2013 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Jun. 30, 2014 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member]	Jun. 30, 2013 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member]	Jun. 30, 2013 Forward Contracts Fair value measured on a recurring basis Level 3 Inputs	Dec. 31, 2012 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, 2014						$ 1,376.7	$ 1,355.1	$ 4,982.2	$ 5,206.1	$ 6,766.5	$ 6,950.7	$ 93.0	$ 92.9	$ 46.1	$ 37.3	$ 6.1	$ 0	$ 262.4	$ 259.3	$ 2,029.3	$ 1,998.2	$ 4,800.3	$ 4,996.6	$ 511.4	$ 407.6	$ 7,341.0	$ 7,402.4	$ 418.7	$ 340.3	$ 4,634.7	$ 4,833.5	$ 158.4	$ 93.0	$ 5,211.8	$ 5,266.8	$ 0	$ 0	$ 59.9	$ 74.4	$ 10.8	$ 6.1	$ 70.7	$ 80.5	$ 0	$ 0	$ 0	$ 0	$ 288.4	$ 262.4			$ 288.4	$ 262.4					$ 537.7	$ 621.5		$ 26.3	$ 14.4	$ 0	$ 0	$ 564.0	$ 635.9
Total realized and unrealized gains						67.3	89.9	89.8	(55.3)	180.7	49.3	0.9	(0.3)	2.7	0.8	3.2	0	16.8	14.2
Foreign currency gains (losses) through OCI						(6.8)	(5.6)	(50.8)	(44.2)	(59.4)	(51.3)	(0.4)	0	0	0	0	0	(1.4)	(1.5)
Amortization/Accretion						(0.3)	(0.6)	(22.0)	(27.0)	(22.4)	(27.6)	(0.1)	0	0	0	0	0	0	0
Purchases						953.5	549.0	1,632.0	2,345.5	2,685.5	2,963.2	76.1	32.3	5.0	0.5	1.5	0	17.4	35.9
Sales						(896.1)	(583.4)	(1,875.6)	(2,713.9)	(2,778.5)	(3,336.0)	0	0	0	0	0	0	(6.8)	(38.7)
Net change in investments related to (sales) purchases of consolidated affiliates						(2.7)	14.5	7.3	2.7	4.6	17.2	0	0	0	0	0	0	0	0
Transfers in						0	1.7	19.6	8.0	28.1	9.7	8.5	0	0	0	0	0	0	0
Transfers out						0	0	(8.5)	(0.7)	(28.1)	(9.7)	(19.6)	(8.0)	0	(1.0)	0	0	0	0
Balance at June 30, 2014						1,491.6	1,420.6	4,774.0	4,721.2	6,777.0	6,565.5	158.4	116.9	53.8	37.6	10.8	0	288.4	269.2	2,029.3	1,998.2	4,800.3	4,996.6	511.4	407.6	7,341.0	7,402.4	418.7	340.3	4,634.7	4,833.5	158.4	93.0	5,211.8	5,266.8	0	0	59.9	74.4	10.8	6.1	70.7	80.5	0	0	0	0	288.4	262.4			288.4	262.4					537.7	621.5		26.3	14.4	0	0	564.0	635.9
Carrying value of investment accounted for using the equity method																																																24.5	26.6	34.2	35.0																						(0.1)	(0.1)
Realized and unrealized gain	90.9	(24.8)	139.0	36.7																																																					0
Net realized and unrealized investment gains (losses)	113.9	(37.3)	177.7	37.9																																																																			1.8	0
Short-term investments, at amortized cost (which approximates fair value)	564.0		564.0		635.9																																																				606.8	635.9	564.0	630.6
Realized and unrealized gains (losses)																																																						(0.1)	0.1	(0.1)
Fixed maturity investments reclassified to assets held for sale					236.3																																																														203.8	236.3	260.8	338.1
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1.0