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Commitments and Contingencies - Legal Narrative (Details) £ in Millions, NZD in Millions |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
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Jan. 26, 2018 USD ($) | Oct. 03, 2017 USD ($) | Oct. 03, 2017 NZD | Jun. 29, 2015 NZD | Jan. 31, 2015 USD ($) | Jan. 26, 2015 USD ($) | Jan. 26, 2015 CHF (SFr) | Dec. 02, 2014 USD ($) | Dec. 02, 2014 CHF (SFr) | Jun. 01, 2007 USD ($) | Jun. 01, 2007 CHF (SFr) | Aug. 31, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2014 CHF (SFr) | May 31, 2010 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 20, 2016 GBP (£) | Apr. 30, 2014 GBP (£)
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,909,000,000 | $ 2,340,000,000 | $ 2,368,000,000 | $ 2,381,000,000 | $ 2,650,000,000 | $ 2,201,000,000 | $ 2,282,000,000 | $ 2,276,000,000 |  | $ 9,998,000,000 | $ 9,409,000,000 | $ 9,480,000,000 |  |
Potential Claim for Pension Advisory Services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60,000,000 |  |  |  |  |  |  |  |  | 60,000,000 |  |  |  | £ 45
Opry Mills Mall Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Opry Mills Mall Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded |  |  |  |  |  |  |  |  |  |  |  | $ 204,000,000 | $ 200,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
International Road Transport Union | Litigation Foreign Currency Denominated Award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  |  |  |  |  |  |  |  | SFr 46,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 47,000,000 |  |  |  |  |
Damages awarded excluding interest and costs |  |  |  |  |  |  |  |  | SFr 16,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17,000,000 |  |  |  |  |
Damages awarded |  |  |  |  |  |  |  |  |  |  |  |  |  |  | SFr 27,900,000 |  |  |  |  |  |  |  |  |  |  | 28,000,000 |  |  |  |
Litigation settlement amount awarded to other party |  |  |  |  | $ 14,000,000 |  | SFr 12,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement terms maximum liability |  |  |  |  |  |  | SFr 15,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000 |  |  |  |  |
International Road Transport Union | Litigation USD Denominated Award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  |  |  |  |  |  |  | $ 3,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded excluding interest and costs |  |  |  |  |  |  |  | $ 3,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Litigation settlement amount awarded to other party |  |  |  |  |  | $ 4,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement terms maximum liability |  |  |  |  |  | $ 344,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
International Road Transport Union | Litigation Expenses and Interest |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  |  |  |  |  |  |  | $ 30,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trustees of Gleeds Pension Fund 2016 | Potential Claim for Pension Advisory Services |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 94,000,000 |  |  |  |  |  |  |  |  | 94,000,000 |  |  | £ 70 |
Pending Litigation | Lyttleton Port Company Limited |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  |  |  | NZD 184 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 130,000,000 |  |  |  |
Pending Litigation | Christchurch City Council |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of damages sought |  | $ 372,000,000 | NZD 528 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  | 0 |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000,000 |  |  |  |  |  |  |  |  | $ 300,000,000 |  |  |  |
Aviation and Aerospace Broking Industry |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |  |  |
Subsequent Event | Opry Mills Mall Limited Partnership |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Damages awarded | $ 50,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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