Shareholders' equity (Detail) $ / shares in Units, $ in Millions		12 Months Ended
	Jan. 14, 2016 shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2015 SFr / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2014 SFr / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2013 SFr / shares shares	May. 31, 2015 $ / shares	Jan. 01, 2015 USD ($)	May. 31, 2014 $ / shares	Nov. 30, 2013 USD ($)	May. 31, 2013 $ / shares
Stockholders' Equity Note [Abstract]
Dividend installments | $ / shares		$ 0.67		$ 0.65		$ 0.51
Increase to dividend approved by shareholders in January | $ / shares				0.12
Common shares in treasury, shares			18,268,971		14,172,726	3,038,477	3,038,477
Conditional Share Issuance, by Scenario [Line Items]
Common Shares, par value | SFr / shares			SFr 24.15		SFr 24.77
Par Value Reduction | (per share)		0.65	0.62	2.46	2.27	$ 2.02	SFr 1.85
Dividends Declared From Additional Paid In Capital | (per share)		$ 2.01	SFr 1.94	0.24	0.20	0	0
Annual dividend per share approved by shareholders | $ / shares								$ 2.68		$ 2.60		$ 2.04
The number of votes associated with one Common Share		1
The maximum ownership percentage for voting allowed for any one shareholder			10.00%
Dividends declared per common share | (per share)		$ 2.66	SFr 2.56	$ 2.70	SFr 2.47	$ 2.02	SFr 1.85
General Purpose
Conditional Share Issuance, by Scenario [Line Items]
Authorized share capital for general purposes			28,048,548
Issuance of Debt
Conditional Share Issuance, by Scenario [Line Items]
Authorized share capital for general purposes			33,000,000
Employee Benefit Plans
Conditional Share Issuance, by Scenario [Line Items]
Authorized share capital for general purposes			25,410,929
November 2013 Stock Repurchase Plan [Member]
Conditional Share Issuance, by Scenario [Line Items]
Repurchase of outstanding common shares, shares		6,677,663		13,982,358		3,266,531
Cost of shares acquired | $		$ 734		$ 1,449		$ 290
Stock repurchase program authorized amount | $											$ 2,000
2015 Stock Repurchase Plan [Member]
Conditional Share Issuance, by Scenario [Line Items]
Stock repurchase program authorized amount | $									$ 1,500
Subsequent Event [Member]
Conditional Share Issuance, by Scenario [Line Items]
Shares issued in connection with Chubb acquisition	137,000,000