(Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended																																																12 Months Ended	9 Months Ended																9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Level 1 Inputs	Sep. 30, 2013 Level 1 Inputs	Sep. 30, 2014 Level 2 Inputs	Sep. 30, 2013 Level 2 Inputs	Sep. 30, 2014 Fair Value	Sep. 30, 2013 Fair Value	Sep. 30, 2014 Fixed maturity investments. Level 3 Inputs	Sep. 30, 2013 Fixed maturity investments. Level 3 Inputs	Sep. 30, 2014 Common equity securities Level 3 Inputs	Sep. 30, 2013 Common equity securities Level 3 Inputs	Sep. 30, 2014 Convertible Debt Securities Level 3 Inputs	Sep. 30, 2013 Convertible Debt Securities Level 3 Inputs	Sep. 30, 2014 Other long-term investments Level 3 Inputs	Sep. 30, 2013 Other long-term investments Level 3 Inputs	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2014 Fair value measured on a recurring basis Fair Value	Dec. 31, 2013 Fair value measured on a recurring basis Fair Value	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2014 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs	Sep. 30, 2013 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs	Sep. 30, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Sep. 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	Sep. 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	Sep. 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	Sep. 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	Sep. 30, 2014 Segment, Discontinued Operations	Dec. 31, 2013 Segment, Discontinued Operations	Sep. 30, 2013 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Sep. 30, 2014 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member]	Sep. 30, 2013 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member]	Sep. 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	$ 1,923.8	$ 1,998.2	$ 4,786.2	$ 4,996.6	$ 446.5	$ 407.6	$ 7,156.5	$ 7,402.4	$ 219.2	$ 340.3	$ 4,670.7	$ 4,833.5	$ 99.5	$ 93.0	$ 4,989.4	$ 5,266.8	$ 0	$ 0	$ 38.0	$ 74.4	$ 10.9	$ 6.1	$ 48.9	$ 80.5	$ 0	$ 0	$ 0	$ 0	$ 297.2	$ 262.4			$ 297.2	$ 262.4					$ 833.9	$ 621.5		$ 1.0	$ 14.4	$ 0	$ 0	$ 834.9	$ 635.9
Total realized and unrealized gains						57.5	140.6	115.0	(70.7)	198.2	80.6	3.1	(3.3)	3.7	0	3.3	0	15.6	14.0
Foreign currency gains (losses) through OCI						(16.8)	1.4	(142.1)	17.5	(163.3)	18.6	(0.9)	0.5	0	0	0	0	(3.5)	(0.8)
Amortization/Accretion						(0.4)	(0.9)	(33.2)	(39.7)	(33.7)	(40.6)	(0.1)	0	0	0	0	0	0	0
Purchases						1,261.6	735.9	2,595.3	3,014.7	3,985.5	3,875.3	80.0	32.4	0	0.5	1.5	0	47.1	91.8
Sales						(1,585.8)	(877.9)	(2,815.2)	(3,309.7)	(4,436.3)	(4,279.4)	0	(0.7)	(10.9)	0	0	0	(24.4)	(91.1)
Net change in investments related to (sales) purchases of consolidated affiliates						(2.7)	14.5	7.4	2.7	4.7	17.2	0	0	0	0	0	0	0	0
Transfers in						0	1.8	75.8	32.1	75.8	124.2	0	90.3	0	0	0	0	0	0
Transfers out						(0.2)	0	0	(91.1)	(75.8)	(124.2)	(75.6)	(32.1)	0	(1.0)	0	0	0	0
Balance at September 30, 2014						1,089.9	1,370.5	4,785.2	4,761.9	6,321.6	6,622.4	99.5	180.0	38.9	36.8	10.9	0	297.2	273.2	1,923.8	1,998.2	4,786.2	4,996.6	446.5	407.6	7,156.5	7,402.4	219.2	340.3	4,670.7	4,833.5	99.5	93.0	4,989.4	5,266.8	0	0	38.0	74.4	10.9	6.1	48.9	80.5	0	0	0	0	297.2	262.4			297.2	262.4					833.9	621.5		1.0	14.4	0	0	834.9	635.9
Carrying value of investment accounted for using the equity method																																																23.5	26.6	26.9	35.0																						(0.1)	(0.1)
Realized and unrealized gain	14.2	27.0	153.1	63.6																																																					0
Net realized and unrealized investment gains	20.8	28.2	198.5	66.1																																																																			0.6	14.1
Short-term investments, at amortized cost (which approximates fair value)	834.9		834.9		635.9																																																				648.0	834.9	635.9	630.6
Realized and unrealized gains (losses)																																																						(0.1)	0.2	(0.1)
Fixed maturity investments reclassified to assets held for sale					236.3																																																														203.9	236.3	239.7	338.1
Maximum price difference provided by pricing services			$ 1.3