Unpaid losses and loss expenses (Narrative) (Details) (USD $)	12 Months Ended				12 Months Ended																								12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2004	Dec. 31, 2010 Insurance - North American	Dec. 31, 2012 Insurance - Overseas General	Dec. 31, 2011 Insurance - Overseas General	Dec. 31, 2010 Insurance - Overseas General	Dec. 31, 2012 Insurance - Overseas General Long tail	Dec. 31, 2011 Insurance - Overseas General Long tail	Dec. 31, 2012 Insurance - Overseas General Long tail Casualty and financial lines 2007 and prior	Dec. 31, 2011 Insurance - Overseas General Long tail Casualty and financial lines 2007 and prior	Dec. 31, 2012 Insurance - Overseas General Long tail Casualty and financial lines 2008 to 2010	Dec. 31, 2011 Insurance - Overseas General Long tail Casualty and financial lines 2008 to 2010	Dec. 31, 2012 Insurance - Overseas General Short tail Property, marine, A&H, and energy lines 2008 to 2009	Dec. 31, 2011 Insurance - Overseas General Short tail Property, marine, A&H, and energy lines 2008 to 2009	Dec. 31, 2012 Global Reinsurance	Dec. 31, 2011 Global Reinsurance	Dec. 31, 2010 Global Reinsurance	Dec. 31, 2012 Global Reinsurance Long tail	Dec. 31, 2011 Global Reinsurance Long tail	Dec. 31, 2012 Global Reinsurance Long tail Other lines	Dec. 31, 2012 Global Reinsurance Long tail Professional liability, D&O, casualty, and medical malpractice 2007 and prior	Dec. 31, 2011 Global Reinsurance Long tail Professional liability, D&O, casualty, and medical malpractice 2007 and prior	Dec. 31, 2012 Global Reinsurance Long tail Non Medical Professional Liability [Member] 2005 and prior	Dec. 31, 2012 Global Reinsurance Short tail 2009 and prior	Dec. 31, 2011 Global Reinsurance Short tail 2009 and prior	Dec. 31, 2012 Penn Millers	Dec. 31, 2012 Brandywine	Dec. 31, 2011 Brandywine	Dec. 31, 2012 Westchester Specialty	Dec. 31, 2011 Westchester Specialty	Dec. 31, 1998 Westchester Specialty	Dec. 31, 2012 Ace Bermuda [Member]	Dec. 31, 2012 Net of NICO Coverage Brandywine	Dec. 31, 2012 Net of NICO Coverage Westchester Specialty	Dec. 31, 2012 Minimum	Dec. 31, 2012 Run Off [Member] Insurance - North American	Dec. 31, 2011 Run Off [Member] Insurance - North American	Dec. 31, 2012 Run Off [Member] Insurance - North American Brandywine and Westchester Specialty runoff	Dec. 31, 2011 Run Off [Member] Insurance - North American Brandywine and Westchester Specialty runoff	Dec. 31, 2012 Run Off [Member] Insurance - North American Completion of reserve review	Dec. 31, 2011 Run Off [Member] Insurance - North American Completion of reserve review	Dec. 31, 2012 Active [Member] Insurance - North American	Dec. 31, 2011 Active [Member] Insurance - North American	Dec. 31, 2012 Active [Member] Insurance - North American Other lines	Dec. 31, 2011 Active [Member] Insurance - North American Other lines	Dec. 31, 2012 Active [Member] Insurance - North American Long tail	Dec. 31, 2011 Active [Member] Insurance - North American Long tail	Dec. 31, 2012 Active [Member] Insurance - North American Long tail Directors and Officers (D&O) Portfolio 2006 and prior	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Directors and Officers (D&O) Portfolio 2006 and prior	Dec. 31, 2012 Active [Member] Insurance - North American Long tail Excess casualty business 2005 and prior	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Excess casualty business 2005 and prior	Dec. 31, 2012 Active [Member] Insurance - North American Long tail Medical risk operations	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Medical risk operations	Dec. 31, 2012 Active [Member] Insurance - North American Long tail National accounts portfolio	Dec. 31, 2011 Active [Member] Insurance - North American Long tail National accounts portfolio	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Financial solutions business 2002 to 2010	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Foreign casualty Products 2007 and prior	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Surety business 2009	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Errors and omissions coverage 2007 to 2008	Dec. 31, 2011 Active [Member] Insurance - North American Long tail Environmental liability product line 2005 to 2007	Dec. 31, 2012 Active [Member] Insurance - North American Short tail	Dec. 31, 2011 Active [Member] Insurance - North American Short tail	Dec. 31, 2012 Active [Member] Insurance - North American Short tail Other lines	Dec. 31, 2011 Active [Member] Insurance - North American Short tail Other lines	Dec. 31, 2012 Active [Member] Insurance - North American Short tail Property portfolios 2009 to 2010	Dec. 31, 2011 Active [Member] Insurance - North American Short tail Property portfolios 2009 to 2010
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	$ 479,000,000	$ 556,000,000	$ 503,000,000		$ 107,000,000	$ 226,000,000	$ 290,000,000	$ 290,000,000	$ 121,000,000	$ 154,000,000	$ 150,000,000	$ 337,000,000	$ (29,000,000)	$ (183,000,000)	$ 105,000,000	$ 136,000,000	$ 61,000,000	$ 71,000,000	$ 106,000,000	$ 32,000,000	$ 58,000,000	$ (4,000,000)	$ 54,000,000	$ 79,000,000	$ (18,000,000)	$ 29,000,000	$ 13,000,000												$ (3,000,000)	$ (168,000,000)	$ (102,000,000)	$ (150,000,000)	$ (82,000,000)	$ (360,000,000)	$ 297,000,000	$ 9,000,000	$ (25,000,000)	$ 245,000,000	$ 186,000,000	$ 67,000,000	$ 82,000,000	$ 73,000,000	$ 54,000,000	$ 57,000,000	$ 43,000,000	$ 39,000,000	$ 28,000,000	$ 26,000,000	$ 26,000,000	$ 21,000,000	$ (40,000,000)	$ (29,000,000)	$ 115,000,000	$ 111,000,000	$ 27,000,000	$ 63,000,000	$ 88,000,000	$ 48,000,000
Prior period net unpaid reserves represented by prior period development (percent)					(0.70%)	(3.10%)	(4.20%)	(4.30%)									(2.70%)	(3.10%)	(4.70%)																					1.00%	(0.60%)			(2.20%)	(1.90%)
Unallocated loss adjustment expenses due to operating expenses paid and reserved	(10,132,000,000)	(10,076,000,000)	(8,082,000,000)																																			(18,000,000)	(17,000,000)
A & E - change in net loss reserves																													146,000,000	76,000,000	4,000,000	6,000,000
A & E - change in gross loss reserves																													275,000,000	241,000,000	17,000,000	(29,000,000)
A & E - loss reserves, net - closing	1,102,000,000	1,304,000,000				84,000,000																						3,000,000	852,000,000		151,000,000			12,000,000
A & E - incurred activity, net	134,000,000																												110,000,000		22,000,000
NICO reinsurance protection on losses and loss expenses																													2,500,000,000
Incurred activity	9,653,000,000	9,520,000,000	7,579,000,000																																129,000,000	4,000,000
NICO pro-rata share of reinsurance protection (percent)																																	75.00%
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/1996, net of retenion																																	1,000,000,000
NICO retention for losses and loss expenses incurred on or before 12/31/1996																																	721,000,000
NICO pro-rata share of reinsurance protection (percent)																																	85.00%
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/1992, net of retenion																																	150,000,000
NICO retention for losses and loss expenses incurred on or before 12/31/1992																																	755,000,000
NICO remaining unused limit under the 1998 agreement																																	492,000,000
Dividend retention fund established by INA Financial Corporation	50,000,000
Required minimum balance under the dividend retention fund	50,000,000
Contributions to the dividend retention fund	0	35,000,000
Minimum contribution from the dividend retention fund to Century not required for XOL agreement	200,000,000
Reinsurance coverage to Century provided by ACE INA under XOL	800,000,000
Statutory surplus of century	25,000,000
Century Statutory Basis Losses Cede To X O L	394,000,000
INA Holdings contibutions to Century in exchange for a surplus note				100,000,000
Aggregate reinsurance balances ceded by active ACE companies to Century	958,000,000	877,000,000
Century's carried gross reserves (including reserves ceded by active ACE companies)	2,100,000,000	2,400,000,000
Minimum number of years related to portion of ceded liabilities																																					25 years
Century remaining NICO cover on a paid loss basis	18,000,000	386,000,000
Aggregate of losses ceded by Century to the active ACE companies and other amounts owed to Century by active ACE companies	$ 402,000,000	$ 171,000,000