Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments Contingencies And Guarantees [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	$ 1
Derivative Liability, Fair Value, Amount Offset Against Collateral		$ 34
Repurchase agreements	1,404	1,402
Securities lending payable	1,047	1,331
Securities lending collateral	1,046	$ 1,330
Approximate percentage of gross premiums written that were generated from or placed by Marsh		10.00%	11.00%
Carrying value of limited partnerships and partially-owned investment companies included in other investments	2,200
Funding commitments relating to limited partnerships and partially-owned investment companies	824
Total rental expense related to operating leases	126	$ 127	$ 128
Purchase Commitment, Remaining Minimum Amount Committed	255
Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,500
Letter Of Credit Unsecured Expiring November 2017 | Letter of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	300
Line of Credit Facility, Amount Outstanding	$ 797