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Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2019 | May 31, 2019 | Dec. 31, 2021 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 31, 2020 | Oct. 25, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  |  | 10.00% |  |
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement | $ 75 |  |  | $ 75 | $ 95 |  |  |
Securities Sold under Agreements to Repurchase | $ 1,416 |  |  | $ 1,416 | $ 1,418 |  |  |
Concentration Risk Percentage Marsh | 12.00% |  |  | 12.00% | 10.00% |  |  |
Purchase Commitment, Remaining Minimum Amount Committed | $ 731 |  |  | $ 731 |  |  |  |
Carrying value of limited partnerships and partially-owned investment companies included in other investments | 4,700 |  |  | 4,700 |  |  |  |
Funding commitments relating to limited partnerships and partially-owned investment companies | 3,300 |  |  | 3,300 |  |  |  |
Line of Credit Facility, Current Borrowing Capacity |  |  |  |  |  |  |  | $ 1,000
Line of Credit Facility, Maximum Borrowing Capacity | 2,000 |  |  | 2,000 |  |  |  |
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases | 1,000 |  |  | 1,000 |  |  |  |
Operating Lease, Right-of-Use Asset | 551 |  |  | 551 |  |  |  |
Operating Lease, Liability | $ 603 |  |  | $ 603 |  |  |  |
Operating Lease, Weighted Average Remaining Lease Term | 5 years 4 months 24 days |  |  | 5 years 4 months 24 days |  |  |  |
Operating Lease, Weighted Average Discount Rate, Percent | 2.70% |  |  | 2.70% |  |  |  |
Total rental expense related to operating leases |  |  |  | $ 171 | $ 169 | $ 211 |  |
Deposit Assets | $ 93 |  |  | 93 | $ 97 |  |  |
Letter of Credit [Member] |  |  |  |  |  |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding | $ 567 |  |  | $ 567 |  |  |  |
Huatai Group [Member] |  |  |  |  |  |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |  |  |  |  |  |
Equity method investment, additional ownership percentage obtained | 4.70% |  |  |  |  |  |  |
Ownership Percentage | 30.90% |  |  | 30.90% | 20.00% |  |  |
CHINA | Huatai Group [Member] |  |  |  |  |  |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |  |  |  |  |  |
Equity method investment, additional ownership percentage obtained |  | 6.20% |  |  |  |  |  |
Ownership Percentage | 31.00% |  |  | 31.00% | 20.00% |  |  |
CHINA | Subsequent Event [Member] | Huatai Group [Member] |  |  |  |  |  |  |  |
Financial Instruments Owned and Pledged as Collateral [Line Items] |  |  |  |  |  |  |  |
Equity method investment, additional 22.4 ownership percentage obtained |  |  | 22.40% |  |  |  |  |
Deposit Assets |  |  |  |  |  |  | $ 1,550 |

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