Commitments, contingencies, and guarantees (Narrative) (Detail) (USD $)	2 Months Ended	12 Months Ended
	Feb. 27, 2014	Dec. 31, 2013 derivative	Dec. 31, 2012	Dec. 31, 2011
Commitments Contingencies And Guarantees [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral		$ (41,000,000)
Derivative Liability, Fair Value, Amount Offset Against Collateral			35,000,000
Securities Sold under Agreements to Repurchase		1,401,000,000	1,401,000,000
Obligation to Return Securities Received as Collateral		1,633,000,000	1,795,000,000
Securities Held as Collateral, at Fair Value		1,632,000,000	1,791,000,000
Repurchase of outstanding common shares, shares	3,437,082	3,266,531	100,000	2,058,860
Number of in-force interest rate swaps		0
Number of in-force credit default swaps		0
Approximate percentage of gross premiums written that were generated from or placed by Marsh		11.00%	11.00%	12.00%
Approximate percentage of gross premiums written that were generated from or placed by Aon				10.00%
Carrying value of limited partnerships and partially-owned investment companies included in other investments		1,900,000,000
Funding commitments relating to limited partnerships and partially-owned investment companies		1,200,000,000
Total rental expense related to operating leases		128,000,000	112,000,000	114,000,000
Cost of shares acquired	326,000,000	290,000,000	7,000,000	132,000,000
Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Line of Credit Facility, Current Borrowing Capacity		1,000,000,000
Letter Of Credit Unsecured Expiring November 2017 | Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Borrowing capacity on unsecured revolving line of credit		1,500,000,000
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases		300,000,000
Line of Credit Facility, Amount Outstanding		376,000,000
Letter Of Credit Unsecured Expiring September 2014 | Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Borrowing capacity on unsecured revolving line of credit		500,000,000
Letter Of Credit Bilateral Uncollateralized Expiring December 2017 | Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Borrowing capacity on unsecured revolving line of credit		425,000,000
Line of Credit Facility, Amount Outstanding		352,000,000
November 2013 Stock Repurchase Plan [Member]
Commitments Contingencies And Guarantees [Line Items]
Share repurchase authorization remaining	$ 1,620,000,000	$ 1,940,000,000