Reinsurance (Details 2) (USD $)	12 Months Ended							12 Months Ended							1 Months Ended	12 Months Ended			19 Months Ended			12 Months Ended																	12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2013 Michigan Catastrophic Claim Association	Jun. 30, 2012 Michigan Catastrophic Claim Association	Dec. 31, 2012 Michigan Catastrophic Claim Association	Dec. 31, 2011 Michigan Catastrophic Claim Association	May 31, 2013 Florida Hurricane Catastrophe Fund	Dec. 31, 2012 Florida Hurricane Catastrophe Fund	Dec. 31, 2011 Florida Hurricane Catastrophe Fund	Dec. 31, 2010 Florida Hurricane Catastrophe Fund	Dec. 31, 2012 National Flood Insurance Program	Dec. 31, 2011 National Flood Insurance Program	Dec. 31, 2010 National Flood Insurance Program	Jun. 30, 2011 Catastrophe reinsurance	Dec. 31, 2012 Catastrophe reinsurance item	Dec. 31, 2011 Catastrophe reinsurance	Dec. 31, 2010 Catastrophe reinsurance	Dec. 31, 2012 Top and drop excess catastrophe reinsurance agreement	Dec. 31, 2012 Top and drop excess catastrophe reinsurance agreement, coverage A	Dec. 31, 2012 Top and drop excess catastrophe reinsurance agreement, coverage B	Dec. 31, 2012 New Jersey Excess Catastrophe Reinsurance agreement item	Dec. 31, 2012 New Jersey agreement, first layer item	Dec. 31, 2012 New Jersey agreement, second layer item	Dec. 31, 2012 New Jersey agreement, contract one item	Dec. 31, 2012 New Jersey agreement, contract two item	Dec. 31, 2012 Kentucky agreement subject to three limits item	Dec. 31, 2012 Kentucky agreement subject to two limits item	Dec. 31, 2012 Pennsylvania agreement subject to three limits item	Dec. 31, 2012 Pennsylvania agreement subject to two limits item	Dec. 31, 2012 Castle Key Insurance Company and its subsidiaries agreements item	Dec. 31, 2012 Catastrophe reinsurance, first five layers item securities	Dec. 31, 2012 Catastrophe reinsurance, sixth layer item	Dec. 31, 2012 Catastrophe reinsurance, existing contracts of sixth layer item	Dec. 31, 2012 Catastrophe reinsurance, additional contracts of sixth layer item	Dec. 31, 2012 Catastrophe reinsurance, seventh layer item	Dec. 31, 2012 Lloyd's of London	Dec. 31, 2011 Lloyd's of London	Dec. 31, 2012 Property-Liability	Dec. 31, 2011 Property-Liability	Dec. 31, 2010 Property-Liability	Dec. 31, 2009 Property-Liability	Dec. 31, 2012 Property-Liability Westport Insurance Corporation	Dec. 31, 2011 Property-Liability Westport Insurance Corporation	Dec. 31, 2012 Discontinued Lines and Coverages	Dec. 31, 2011 Discontinued Lines and Coverages
Reinsurance recoverable
Total amounts recoverable from reinsurers, industry pools or facilities	$ 8,767,000,000	$ 7,251,000,000				$ 2,590,000,000	$ 1,710,000,000					$ 428,000,000	$ 33,000,000																								$ 190,000,000	$ 193,000,000	$ 4,080,000,000	$ 2,670,000,000
Reinsurance recoverables related to losses paid by the Company and billed to reinsurers																																							69,000,000	86,000,000
Reinsurance recoverables related to ceded unpaid losses																																							4,010,000,000	2,588,000,000	2,072,000,000	2,139,000,000
Largest reinsurance recoverable from a single reinsurer																																											95,000,000	98,000,000
Maximum amount due from any single reinsurer not separately disclosed																																							42,000,000	36,000,000
Allowance for uncollectible reinsurance																																													87,000,000	103,000,000
Retention level per claim				500,000	500,000
Ceded premiums earned	1,090,000,000	1,098,000,000	1,092,000,000						18,000,000	27,000,000	15,000,000	311,000,000	312,000,000	306,000,000		531,000,000	531,000,000	582,000,000
Ceded losses incurred	2,051,000,000	927,000,000	271,000,000							8,000,000	10,000,000	758,000,000	196,000,000	50,000,000
Percent of losses in excess of retention level to be reimbursed								90.00%
Retention level for losses related to 2 largest hurricanes								89,000,000
Retention level for losses related to other hurricanes								30,000,000
Maximum Retention level for losses related to hurricanes								236,000,000
Contracts number																9						3						1		1		3	5	2	3	1
Retention percentage of reinsurance limit																				12.67%	25.32%		32.00%	42.00%	31.66%	31.67%		95.00%	95.00%			95.00%	82.33%			83.12%
Limit of aggregate losses under aggregate excess agreement																			250,000,000	500,000,000	250,000,000		300,000,000	200,000,000	400,000,000	400,000,000	25,000,000		100,000,000		666,000,000	3,250,000,000	500,000,000			475,000,000
Number of separate catastrophe reinsurance agreements coordinating coverage for excess losses for Castle Key																															5
Term of catastrophe reinsurance agreement contract one															1 year
Term of catastrophe reinsurance agreement contract two															2 years
Term of catastrophe reinsurance agreement contract three															3 years
Number of layers in which catastrophe reinsurance agreement contracts are placed																7
Number of layers subject to reinstatement in which catastrophe reinsurance agreement contracts are placed																																5
Retention amount for catastrophe reinsurance agreement contracts																				3,250,000,000	750,000,000		171,000,000	471,000,000	139,000,000	150,000,000	5,000,000		100,000,000		30,000,000	500,000,000	3,250,000,000			3,750,000,000
Number of reinstatements each year for each contract																							1	1	1	1
Losses otherwise recoverable as per catastrophe reinsurance agreement contracts																					$ 500,000,000											$ 250,000,000
Number of limits																											3	2	3	2