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Claims, Lawsuits, and Other Contingencies - Legal Narrative (Details) £ in Millions, $ in Millions |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 06, 2018 USD ($) | Jan. 26, 2018 USD ($) | Oct. 03, 2017 NZD ($) | Jun. 29, 2015 NZD ($) | Sep. 14, 2010 USD ($) | Aug. 31, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 20, 2016 GBP (£) | Apr. 30, 2014 GBP (£)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  | $ 2,770,000,000 | $ 2,349,000,000 | $ 2,561,000,000 | $ 3,090,000,000 | $ 2,909,000,000 | $ 2,340,000,000 | $ 2,368,000,000 | $ 2,381,000,000 | $ 10,770,000,000 | $ 9,998,000,000 | $ 9,409,000,000 |  |
Potential Claim for Pension Advisory Services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  | 57,000,000 |  |  |  |  |  |  |  | 57,000,000 |  |  |  | £ 45
Trustees of Gleeds Pension Fund 2016 | Potential Claim for Pension Advisory Services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  | 88,000,000 |  |  |  |  |  |  |  | 88,000,000 |  |  | £ 70 |
Opry Mills Mall Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  |  | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded |  | $ 50,000,000 |  |  |  | $ 204,000,000 | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of coverage for damages contended by the insurers |  |  |  |  | 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Difference amount of damages sought by the client |  |  |  |  | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pending Litigation | Lyttleton Port Company Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  | $ 184 |  |  |  |  |  |  |  |  |  |  |  | 124,000,000 |  |  |  |
Pending Litigation | Christchurch City Council |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  | $ 528 |  |  |  |  |  |  |  |  |  |  |  |  | 355,000,000 |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  | 0 |  |  |  |
Minimum | Pilkington North America, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought | $ 45,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  | $ 200,000,000 |  |  |  |  |  |  |  | $ 200,000,000 |  |  |  |
Maximum | Pilkington North America, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought | 85,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded | $ 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aviation and Aerospace Broking Industry |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |  |

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