Unpaid losses and loss expenses Unpaid losses and loss expenses (PPD table) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 14, 2016	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net		$ 49,165	$ 47,832	$ 26,562		$ 27,008
Prior Year Claims and Claims Adjustment Expense		$ (829)	$ (1,135)	$ (546)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.70%	2.40%	2.00%
Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (423)	$ (817)	$ (263)
Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(406)	(318)	(283)
North America Commercial P&C Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (746)	$ (778)	$ (264)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	1.60%	1.60%	1.00%
North America Commercial P&C Insurance | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (562)	$ (693)	$ (162)
North America Commercial P&C Insurance | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(184)	(85)	(102)
North America Personal P&C Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 69	$ 27	$ 25
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.10%	0.10%	0.10%
North America Personal P&C Insurance [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 0	$ 0	$ 0
North America Personal P&C Insurance [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		69	27	25
North America Agricultural Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (119)	$ (72)	$ (45)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.20%	0.20%	0.10%
North America Agricultural Insurance | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 0	$ 0	$ 0
North America Agricultural Insurance | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(119)	(72)	(45)
Overseas General Insurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (252)	$ (423)	$ (343)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.50%	0.90%	1.30%
Overseas General Insurance | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (71)	$ (236)	$ (192)
Overseas General Insurance | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(181)	(187)	(151)
Global Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (59)	$ (78)	$ (119)
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.10%	0.20%	0.40%
Global Reinsurance | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ (68)	$ (77)	$ (109)
Global Reinsurance | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		9	(1)	(10)
Corporate Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 278	$ 189	$ 200
Prior Period Development Percentage Opening Net Unpaid Reserves	[1]	0.60%	0.40%	0.70%
Corporate Segment [Member] | Long Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 278	$ 189	$ 200
Corporate Segment [Member] | Short Tail [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 0	$ 0	$ 0
The Chubb Corporation [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net					$ 21,400

[1] (1) Calculated based on the beginning of period consolidated net unpaid losses and loss expenses. For 2016, the percent of beginning net unpaid reserves is calculated inclusive of the net unpaid losses and loss expenses acquired in the Chubb Corp acquisition of $21.4 billion.