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Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning Balance |  |  | $ 4,648 |  |  |
Adoption of new accounting guidance |  |  |  |  | $ 492 |
Balance at January 1, 2018 |  |  |  |  | 5,140 | $ 5,581
Other comprehensive income (loss) before reclassifications, net |  |  | (231) |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 123 |  |  |
Tax expense |  |  | (27) |  |  |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 96 |  |  |
Total other comprehensive income (loss) attributable to Aon shareholders | $ (108) | $ 265 | (135) | $ 500 |  |
Ending Balance | 4,328 | $ 5,247 | 4,328 | $ 5,247 |  |
Total |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning Balance |  |  | (3,496) |  |  |
Balance at January 1, 2018 |  |  |  |  | (3,497) |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |  |
Ending Balance | (3,632) |  | (3,632) |  |  |
Change in Fair Value of Financial Instruments |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning Balance |  |  | (25) |  |  |
Balance at January 1, 2018 |  |  |  |  | (26) |
Other comprehensive income (loss) before reclassifications, net |  |  | 7 |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 9 |  |  |
Tax expense |  |  | (2) |  |  |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 7 |  |  |
Total other comprehensive income (loss) attributable to Aon shareholders |  |  | 14 |  |  |
Ending Balance | (12) |  | (12) |  |  |
Foreign Currency Translation Adjustments |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning Balance |  |  | (879) |  |  |
Balance at January 1, 2018 |  |  |  |  | (879) |
Other comprehensive income (loss) before reclassifications, net |  |  | (257) |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 0 |  |  |
Tax expense |  |  | 0 |  |  |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 0 |  |  |
Total other comprehensive income (loss) attributable to Aon shareholders |  |  | (257) |  |  |
Ending Balance | (1,136) |  | (1,136) |  |  |
Postretirement Benefit Obligation |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning Balance |  |  | (2,592) |  |  |
Balance at January 1, 2018 |  |  |  |  | (2,592) |
Other comprehensive income (loss) before reclassifications, net |  |  | 19 |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 114 |  |  |
Tax expense |  |  | (25) |  |  |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 89 |  |  |
Total other comprehensive income (loss) attributable to Aon shareholders |  |  | 108 |  |  |
Ending Balance | $ (2,484) |  | $ (2,484) |  |  |
Accounting Standards Update 2016-01 | Total |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  | (1) |
Accounting Standards Update 2016-01 | Change in Fair Value of Financial Instruments |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  | (1) |
Accounting Standards Update 2016-01 | Foreign Currency Translation Adjustments |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  | 0 |
Accounting Standards Update 2016-01 | Postretirement Benefit Obligation |  |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |  |
Adoption of new accounting guidance |  |  |  |  | $ 0 |

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