Commitments, contingencies, and guarantees (Narrative) (Detail) (USD $)	12 Months Ended								2 Months Ended
	Dec. 31, 2012 derivative	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Revolving Credit Facility [Member]	Dec. 31, 2012 Letter of Credit [Member]	Dec. 31, 2012 Letter Of Credit Unsecured Expiring November 2017 Letter of Credit [Member]	Dec. 31, 2012 Letter Of Credit Unsecured Expiring September 2014 Letter of Credit [Member]	Dec. 31, 2012 Letter Of Credit Bilateral Uncollateralized Expiring December 2017 Letter of Credit [Member]	Feb. 27, 2013 Subsequent Event [Member]	Feb. 27, 2013 November 2011 Stock Repurchase Plan [Member] Subsequent Event [Member]
Commitments Contingencies And Guarantees [Line Items]
Repurchase of outstanding common shares, shares	100,000	2,058,860	4,926,082							1,746,123
Number of positions in TBAs	0
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%	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	$ 1,700,000,000
Funding commitments relating to limited partnerships and partially-owned investment companies	1,200,000,000
Line of Credit Facility, Current Borrowing Capacity					1,000,000,000
Borrowing capacity on unsecured revolving line of credit						1,500,000,000	500,000,000	425,000,000
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases				500,000,000		300,000,000
Line of Credit Facility, Amount Outstanding						619,000,000		400,000,000
Litigation Settlement, Gross	4,200,000
Total rental expense related to operating leases	112,000,000	114,000,000	83,000,000
Cost of shares acquired	7,000,000	132,000,000	303,000,000							149,000,000
Share repurchase authorization remaining	$ 461,000,000								$ 312,000,000