Discontinued Operations (Additional Disclosures) (Details) (USD $)	3 Months Ended						3 Months Ended																				3 Months Ended	12 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2014 Segment, Discontinued Operations	Mar. 31, 2013 Segment, Discontinued Operations	Dec. 31, 2013 Segment, Discontinued Operations rating	Dec. 31, 2013 Segment, Discontinued Operations GRC Cover	May 31, 2001 Segment, Discontinued Operations GRC Cover	Mar. 31, 2014 Segment, Discontinued Operations NICO Cover	Dec. 31, 2013 Segment, Discontinued Operations NICO Cover	Mar. 31, 2014 Segment, Discontinued Operations National Indemnity Company (NICO) and General Reinsurance Corporation		Dec. 31, 2013 Segment, Discontinued Operations National Indemnity Company (NICO) and General Reinsurance Corporation	Mar. 31, 2014 Segment, Discontinued Operations Hanover Insurance Company	Mar. 31, 2014 Segment, Discontinued Operations Tokio Marine and Nichido Fire		Dec. 31, 2013 Segment, Discontinued Operations Tokio Marine and Nichido Fire	Mar. 31, 2014 Segment, Discontinued Operations Munich Reinsurance America	Mar. 31, 2014 Segment, Discontinued Operations Tower Insurance Company	Mar. 31, 2014 Runoff	Dec. 31, 2013 Runoff	Mar. 31, 2013 Runoff	Dec. 31, 2012 Runoff	Mar. 31, 2014 Runoff Segment, Discontinued Operations	Dec. 31, 2013 Runoff Segment, Discontinued Operations	Dec. 31, 2001 General Reinsurance Corporation Cover Segment, Discontinued Operations GRC Cover
Dispositions
Quota Share Reinsurance Agreement Percentage						100.00%
Reinsurance Recoverables, Gross							$ 133,600,000		$ 136,900,000		$ 570,000,000
Reinsurance recoverable on unpaid losses	80,100,000		105,800,000	80,200,000	107,300,000		1,673,600,000		1,741,600,000			2,500,000,000															1,540,000,000	1,604,700,000
Unpaid loss and loss adjustment expense reserves	1,069,800,000		1,010,300,000	1,054,300,000	1,000,000,000		1,850,900,000		1,930,000,000																		1,717,300,000	1,793,100,000
Pre-tax income (loss)							(800,000)	700,000																			69,000,000	69,000,000
Income (loss) from discontinued operations, net of tax	(500,000)		500,000				(500,000)	500,000
Income (loss) from discontinued operations, net of tax							(500,000)	500,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	989,700,000		904,500,000	974,100,000	892,700,000																			177,300,000
Loss and loss adjustment expenses	149,400,000		148,900,000				0	0
Loss and LAE incurred relating to prior year losses	(1,400,000)		(2,900,000)
Adverse loss reserve development on losses occurring in years 2000 and prior																													400,000,000
Loss reserves ceded at acquisition																													170,000,000
LAE claims, net												2,300,000,000
Reinsurance recoverables	83,100,000	[1]		89,900,000						359,700,000		1,700,000,000		1,188,800,000	[2]		38,700,000	26,100,000	[3]		14,100,000	9,900,000
LAE claims, estimated future losses													198,300,000
Estimated incurred losses ceded										562,000,000
Reinsurance recoverable on paid losses	3,000,000																										9,500,000	10,700,000
Percentage of total paid and unpaid reinsurance recoverables	100.00%	[1]												71.00%	[2]		2.00%	2.00%	[3]		1.00%	1.00%
Number of ratings in A.M. Best ratings									16
Reinsurance recoverables																198,300,000				21,400,000
Loss and Loss Adjustment Expense Reserves Reclassified to from Held For Sale																							$ (177,300,000)	$ 188,400,000	$ (166,100,000)	$ 211,800,000

[1] Standard & Poor's ratings as detailed above are "AA" (Very strong), "A" (Strong) and "BBB+" (Adequate).

[2] Includes $198.3 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover if they are unable to collect from third party reinsurers.

[3] Excludes $21.4 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.