Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement	$ 24	$ 10
Securities lending collateral	1,737	1,092
Collateral held under securities lending agreements	1,737	1,093
Purchase Commitment, Remaining Minimum Amount Committed	1,020
Carrying value of limited partnerships and partially-owned investment companies included in other investments	3,400
Funding commitments relating to limited partnerships and partially-owned investment companies	4,100
Line of Credit Facility, Current Borrowing Capacity	1,000			$ 1,500
Line of Credit Facility, Maximum Borrowing Capacity	2,000
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	1,000
Total rental expense related to operating leases	211	209	$ 126
Securities Sold under Agreements to Repurchase	$ 1,408	$ 1,403
Concentration Risk, Percentage	10.00%
Letter of Credit [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Line of Credit Facility, Amount Outstanding	$ 250