Unpaid Losses And Loss Expenses (Narrative) (Details) (USD $)	12 Months Ended						12 Months Ended				12 Months Ended		12 Months Ended												12 Months Ended																								12 Months Ended												12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2010	Dec. 31, 2004	Jun. 30, 2010 LOB Brandywine Run Off [Member]	Dec. 31, 2011 Insurance - North American [Member]	Dec. 31, 2010 Insurance - North American [Member]	Dec. 31, 2009 Insurance - North American [Member]	Dec. 31, 2008 Insurance - North American [Member]	Dec. 31, 2011 Insurance - North American [Member] Run Off [Member]	Dec. 31, 2010 Insurance - North American [Member] Run Off [Member]	Dec. 31, 2010 Insurance - North American [Member] LOB Brandywine And Westchester Specialty Run Off [Member] Underwriting Period Reserve Review [Member] Run Off [Member]	Dec. 31, 2011 Insurance - North American [Member] LOB Completion Of Reserve Review [Member] Underwriting Period 2011 [Member] Run Off [Member]	Dec. 31, 2011 Insurance - North American [Member] Short Tail [Member]	Dec. 31, 2010 Insurance - North American [Member] Short Tail [Member] Active [Member]	Dec. 31, 2010 Insurance - North American [Member] Short Tail [Member] LOB Crop Hail [Member] Underwriting Period 2009 And Prior [Member]	Dec. 31, 2011 Insurance - North American [Member] Short Tail [Member] LOB Property [Member] Underwriting Period 2009 To 2010 [Member]	Dec. 31, 2010 Insurance - North American [Member] Short Tail [Member] LOB Other Lines [Member] Underwriting Period 2007 To 2009 [Member]	Dec. 31, 2011 Insurance - North American [Member] Short Tail [Member] LOB Other Lines [Member] Underwriting Period Various [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] Run Off [Member]	Dec. 31, 2009 Insurance - North American [Member] Long Tail [Member] Run Off [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Excess Casualty [Member] Underwriting Period 2005 And Prior [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Excess Casualty [Member] Underwriting Period 2007 [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Workers Compensation [Member] Underwriting Period 2008 [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Brandywine And Westchester Specialty Run Off [Member] Underwriting Period 1999 And Prior [Member] Run Off [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Brandywine And Westchester Specialty Run Off [Member] Underwriting Period 2000 And Prior [Member] Run Off [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Brandywine And Westchester Specialty Run Off [Member] Underwriting Period 2011 [Member] Run Off [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Financial Solutions [Member] Underwriting Period 2000 And Prior [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Financial Solutions [Member] Underwriting Period 2002 To 2010 [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Errors And Omission And Directors And Officers [Member] Underwriting Period 2006 And Prior [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] D & O Portfolio [Member] Underwriting Period 2006 And Prior [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Errors And Omissions Coverage [Member] Underwriting Period 2007 To 2008 [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Medical Risk Operations [Member] Underwriting Period 2002 To 2010 [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB National Accounts [Member] Underwriting Period 2005 And 2006 And 2009 [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB National Accounts [Member] Underwriting Period 2002 To 2010 [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Foreign Casualty Products [Member] Underwriting Period 2007 And Prior [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Surety Business [Member] Underwriting Period 2007 [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Environmental Liability Product Line [Member] Underwriting Period 2005 To 2007 [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Other Lines [Member] Underwriting Period Various [Member]	Dec. 31, 2010 Insurance - North American [Member] Long Tail [Member] LOB Other Lines [Member] Underwriting Period Various [Member]	Dec. 31, 2011 Insurance - North American [Member] Long Tail [Member] LOB Other Lines [Member] Underwriting Period Various [Member] Run Off [Member]	Dec. 31, 2011 Insurance - Overseas General [Member]	Dec. 31, 2010 Insurance - Overseas General [Member]	Dec. 31, 2009 Insurance - Overseas General [Member]	Dec. 31, 2008 Insurance - Overseas General [Member]	Dec. 31, 2011 Insurance - Overseas General [Member] Short Tail [Member] LOB Property [Member] Underwriting Period 2007 To 2009 [Member]	Dec. 31, 2011 Insurance - Overseas General [Member] Short Tail [Member] LOB Property Energy Marine Accident And Health [Member] Underwriting Period 2008 To 2009 [Member]	Dec. 31, 2011 Insurance - Overseas General [Member] Long Tail [Member]	Dec. 31, 2010 Insurance - Overseas General [Member] Long Tail [Member]	Dec. 31, 2010 Insurance - Overseas General [Member] Long Tail [Member] LOB Casualty And Financial Lines [Member] Underwriting Period 2006 And Prior [Member]	Dec. 31, 2011 Insurance - Overseas General [Member] Long Tail [Member] LOB Casualty And Financial Lines [Member] Underwriting Period 2007 And Prior [Member]	Dec. 31, 2010 Insurance - Overseas General [Member] Long Tail [Member] LOB Casualty And Financial Lines [Member] Underwriting Period 2007 To 2009 [Member]	Dec. 31, 2011 Insurance - Overseas General [Member] Long Tail [Member] LOB Casualty And Financial Lines [Member] Underwriting Period 2008 To 2010 [Member]	Dec. 31, 2011 Segment Global Reinsurance [Member]	Dec. 31, 2010 Segment Global Reinsurance [Member]	Dec. 31, 2009 Segment Global Reinsurance [Member]	Dec. 31, 2008 Segment Global Reinsurance [Member]	Dec. 31, 2011 Segment Global Reinsurance [Member] Short Tail [Member] Underwriting Period 2009 And Prior [Member]	Dec. 31, 2010 Segment Global Reinsurance [Member] Short Tail [Member] LOB Property And Trade Credit [Member] Underwriting Period 2003 To 2008 [Member]	Dec. 31, 2011 Segment Global Reinsurance [Member] Long Tail [Member]	Dec. 31, 2010 Segment Global Reinsurance [Member] Long Tail [Member]	Dec. 31, 2010 Segment Global Reinsurance [Member] Long Tail [Member] Underwriting Period 2003 To 2006 [Member]	Dec. 31, 2011 Segment Global Reinsurance [Member] Long Tail [Member] Professional Liability, D & O, Casualty, And Medical Malpractice [Member]	Dec. 31, 2011 Brandywine [Member]	Dec. 31, 2010 Brandywine [Member]	Dec. 31, 2011 Westchester Specialty [Member]	Dec. 31, 1998 Westchester Specialty [Member]	Dec. 31, 2011 ACE Bermuda [Member]
Unpaid Losses And Loss Expenses [Line Items]
Net losses and loss expenses incurred, Prior years	$ (556,000,000)	$ (503,000,000)	$ (576,000,000)				$ 297,000,000	$ 239,000,000	$ 179,000,000				$ 89,000,000	$ 82,000,000	$ 111,000,000	$ 137,000,000	$ 41,000,000	$ 48,000,000	$ 96,000,000	$ 63,000,000	$ 186,000,000	$ 102,000,000	$ 132,000,000		$ 54,000,000	$ 91,000,000	$ 30,000,000	$ 114,000,000	$ 18,000,000	$ 102,000,000	$ 49,000,000	$ 26,000,000	$ 105,000,000	$ 82,000,000	$ 40,000,000	$ 43,000,000	$ 54,000,000	$ 28,000,000	$ 26,000,000	$ 21,000,000	$ 29,000,000	$ 25,000,000	$ 15,000,000	$ 3,000,000	$ 290,000,000	$ 290,000,000	$ 255,000,000		$ 131,000,000	$ 136,000,000	$ 154,000,000	$ 159,000,000	$ 241,000,000	$ 337,000,000	$ 82,000,000	$ 183,000,000	$ 71,000,000	$ 106,000,000	$ 142,000,000		$ 13,000,000	$ 34,000,000	$ 58,000,000	$ 72,000,000	$ 96,000,000	$ 79,000,000
Percent of prior period net unpaid reserves represented by prior period development								1.90%	1.50%	1.20%		0.60%												0.80%																						4.20%	4.30%	4.20%										3.10%	4.70%	5.60%
Unallocated loss adjustment expenses due to operating expenses paid and reserved	10,076,000,000	8,082,000,000	7,998,000,000								17,000,000
A&E - change in net loss reserves																																																																			76,000,000	84,000,000	6,000,000
A&E - change in gross loss reserves																																																																			241,000,000	247,000,000	29,000,000
A&E - loss reserves, net - closing	1,196,000,000	1,335,000,000																																											83,000,000																						909,000,000		143,000,000		61,000,000
A&E net - incurred activity	104,000,000																																												3,000,000																						59,000,000		48,000,000
NICO reinsurance protection on losses and loss expenses	2,500,000,000
NICO % reinsurance protection on losses and loss expenses incurred on or before 12/31/96 net of retention																																																																						75.00%
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/96 net of retention																																																																						1,000,000,000
NICO retention for losses and loss expenses incurred on or before 12/31/96																																																																						721,000,000
NICO pro-rata share of reinsurance protection before 12/31/92																																																																						85.00%
NICO reinsurance protection on losses and loss expenses incurred on or before 12/31/92 net of retention																																																																						150,000,000
NICO retention for losses and loss expenses incurred on or before 12/31/92																																																																						755,000,000
NICO remaining unused limit under the 1998 agreement																																																																						504,000,000
Statutory surplus required by Century under the 1996 Pennsylvania insurance department restructuring order						25,000,000
Dividend retention fund established by INA Financial Corporation	50,000,000
The minimum required balance of the Dividend Retention Fund	50,000,000
Contributions to the dividend retention fund	35,000,000	15,000,000
Minimum capital contributions from the dividend retention fund to century not be required for xol agreement	200,000,000
Increase in Century's assets as a result of a transfer in CIRC stock	169,000,000
Increase in Century's surplus as a result of a transfer in CIRC stock				26,000,000
Statutory surplus of century	25,000,000			51,000,000
Aggregate reinsurance balances ceded by active ACE entities to Century	877,000,000	881,000,000
Gross reserves carried by Century	2,400,000,000	2,700,000,000
Minimum number of years for which liabilities relating to intercompany recoverables are payable	25
Remaining cover on a paid loss basis after being ceded from Century to NICO	624,000,000	928,000,000
Losses ceded by Century to the active ACE Companies and other amounts owed to Century	526,000,000	453,000,000
INA Holdings contribution to Century in exchange for a surplus note					100,000,000
Statutory-basis losses ceded to the XOL agreement on an inception to date basis	146,000,000
Amount of undiscounted losses ceded under the XOL agreement	440,000,000
Remaining limit of coverage under the XOL agreement	360,000,000		410,000,000
Amount of excess of loss (XOL) reinsurance coverage provided to Century	800,000,000
Minimum capital and surplus for the XOL not to be triggered	$ 25,000,000