Unpaid losses and loss expenses (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2004
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	$ 527,000,000		$ 530,000,000	$ 479,000,000
Unallocated loss adjustment expenses due to operating expenses paid and reserved	(10,176,000,000)		(9,878,000,000)	(10,132,000,000)
A & E - loss reserves, net - closing	1,041,000,000		1,051,000,000
A & E - incurred activity, net	210,000,000	[1]	171,000,000
NICO reinsurance protection on losses and loss expenses	472,000,000
Incurred activity	9,649,000,000		9,348,000,000	9,653,000,000
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)	75.00%
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					35,000,000	15,000,000
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	298,000,000
INA Holdings contibutions to Century in exchange for a surplus note							100,000,000
Aggregate reinsurance balances ceded by active ACE companies to Century	1,100,000,000		929,000,000
Century's carried gross reserves (including reserves ceded by active ACE companies)	2,100,000,000		2,300,000,000
Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	391,000,000		299,000,000	226,000,000
Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	63,000,000		84,000,000	61,000,000
Long tail | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	181,000,000		127,000,000
Long tail | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	52,000,000		53,000,000
Long tail | Excess casualty business | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			92,000,000
Long tail | Excess casualty business | 2010 to 2012 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			(43,000,000)
Long tail | Excess casualty business | 2009 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			135,000,000
Long tail | Casualty | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	102,000,000
Long tail | Casualty | 2011 to 2013 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(46,000,000)
Long tail | Casualty | 2010 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	148,000,000
Long tail | Casualty | 2009 and prior | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	25,000,000
Long tail | Medical risk operations | 2009 and prior | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			20,000,000
Long tail | Financial solutions business | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	27,000,000		35,000,000
Long tail | Financial solutions business | 2011 to 2013 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(71,000,000)
Long tail | Financial solutions business | 2010 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	98,000,000
Long tail | Financial solutions business | 2009 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			63,000,000
Long tail | Financial solutions business | 2012 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			(28,000,000)
Long tail | Other lines | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	52,000,000
Long tail | Professional liability, including medical malpractice | 2009 and prior | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	34,000,000
Long tail | Non Medical Professional Liability [Member] | 2008 and prior | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			25,000,000
Short tail | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	210,000,000		172,000,000
Short tail | Other lines | Insurance - North American Agriculture
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(34,000,000)		13,000,000	12,000,000
Short tail | Property portfolios | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			104,000,000
Short tail | Property portfolios | 2007 to 2012 | Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			31,000,000
Short tail | Property portfolios | 2010 to 2012 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			69,000,000
Short tail | Property portfolios | 2009 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			35,000,000
Short tail | Accident & Health and Personal Lines | 2010 to 2012 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			39,000,000
Short tail | Personal Lines | 2011 to 2013 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	25,000,000
Short tail | Property and inland marine lines | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	136,000,000
Short tail | Property and inland marine lines | 2012 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	92,000,000
Short tail | Property and inland marine lines | 2013 | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	44,000,000
Short tail | Aviation lines [Domain] | 2010 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	30,000,000
Short tail | Aviation lines [Domain] | 2009 and prior | Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			29,000,000
Insurance - Overseas General
Unpaid Losses and Loss Expenses [Line Items]
Prior period net unpaid reserves represented by prior period development (percent)				(3.10%)
Global Reinsurance
Unpaid Losses and Loss Expenses [Line Items]
Prior period net unpaid reserves represented by prior period development (percent)				(2.70%)
Brandywine
Unpaid Losses and Loss Expenses [Line Items]
A & E - loss reserves, net - closing	837,000,000		816,000,000
Westchester Specialty
Unpaid Losses and Loss Expenses [Line Items]
A & E - loss reserves, net - closing	119,000,000		146,000,000
Ace Bermuda [Member]
Unpaid Losses and Loss Expenses [Line Items]
A & E - loss reserves, net - closing	85,000,000		89,000,000
Run Off [Member] | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(247,000,000)		(193,000,000)	(168,000,000)
Run Off [Member] | Completion of reserve review | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(215,000,000)		(161,000,000)
Run Off [Member] | unallocated LAE - run-off | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(32,000,000)		(27,000,000)
Run Off [Member] | Insurance - North American
Unpaid Losses and Loss Expenses [Line Items]
Prior period net unpaid reserves represented by prior period development (percent)				(1.10%)
Active [Member] | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	354,000,000		327,000,000	348,000,000
Active [Member] | Long tail | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	298,000,000		221,000,000
Active [Member] | Long tail | Directors and Officers (D&O) Portfolio | 2009 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	104,000,000
Active [Member] | Long tail | Directors and Officers (D&O) Portfolio | 2008 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			72,000,000
Active [Member] | Long tail | Excess casualty business | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	55,000,000
Active [Member] | Long tail | Excess casualty business | 2007 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			50,000,000
Active [Member] | Long tail | Excess casualty business | 2003 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	42,000,000
Active [Member] | Long tail | Medical risk operations | 2007 to 2009 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			63,000,000
Active [Member] | Long tail | Medical risk operations | 2009 to 2010 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	40,000,000
Active [Member] | Long tail | National accounts portfolio | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			28,000,000
Active [Member] | Long tail | National accounts portfolio | 2012 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			40,000,000
Active [Member] | Long tail | Financial solutions business | 2010 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	35,000,000
Active [Member] | Long tail | Foreign casualty Products | 2009 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			25,000,000
Active [Member] | Long tail | Surety business | 2012 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	27,000,000
Active [Member] | Long tail | Other lines | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	48,000,000		28,000,000
Active [Member] | Long tail | Auto Liability Excess Lines | 2009 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	21,000,000
Active [Member] | Long tail | Workers Compensation [Domain] | 2006 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			(40,000,000)
Active [Member] | Long tail | Workers Compensation [Domain] | 2013 and prior | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	(32,000,000)
Active [Member] | Short tail | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	56,000,000		106,000,000
Active [Member] | Short tail | Energy and Technical Risk Property | 2012 to 2013 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years	20,000,000
Active [Member] | Short tail | Property and inland marine lines | 2010 to 2012 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			45,000,000
Active [Member] | Short tail | Political risk lines [Domain] | 2009 to 2010 | Insurance - North American P&C
Unpaid Losses and Loss Expenses [Line Items]
Net losses and loss expenses incurred in prior years			$ 29,000,000
Active [Member] | Insurance - North American
Unpaid Losses and Loss Expenses [Line Items]
Prior period net unpaid reserves represented by prior period development (percent)				(2.20%)

[1] (1)Excludes unallocated loss expenses and the net activity reflects third-party reinsurance other than the aggregate excess of loss reinsurance provided by National Indemnity Company (NICO) to Westchester Specialty (see Westchester Specialty section below).