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Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Beginning Balance | $ 3,913 | $ 4,618 | $ 4,219 | $ 5,140
Other comprehensive income (loss) before reclassifications, net |  |  | (190) |
Amounts reclassified from accumulated other comprehensive income |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 91 |
Tax expense |  |  | (22) |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 69 |
Total other comprehensive loss attributable to Aon shareholders | (193) | (108) | (121) | (135)
Ending Balance | 3,561 | $ 4,328 | 3,561 | $ 4,328
Total |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | (3,909) |
Amounts reclassified from accumulated other comprehensive income |  |  |  |
Ending Balance | (4,030) |  | (4,030) |
Change in Fair Value of Financial Instruments |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | (15) |
Other comprehensive income (loss) before reclassifications, net |  |  | (12) |
Amounts reclassified from accumulated other comprehensive income |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 12 |
Tax expense |  |  | (3) |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 9 |
Total other comprehensive loss attributable to Aon shareholders |  |  | (3) |
Ending Balance | (18) |  | (18) |
Foreign Currency Translation Adjustments |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | (1,319) |
Other comprehensive income (loss) before reclassifications, net |  |  | (180) |
Amounts reclassified from accumulated other comprehensive income |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 0 |
Tax expense |  |  | 0 |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 0 |
Total other comprehensive loss attributable to Aon shareholders |  |  | (180) |
Ending Balance | (1,499) |  | (1,499) |
Postretirement Benefit Obligation |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |
Beginning Balance |  |  | (2,575) |
Other comprehensive income (loss) before reclassifications, net |  |  | 2 |
Amounts reclassified from accumulated other comprehensive income |  |  |  |
Amounts reclassified from accumulated other comprehensive income |  |  | 79 |
Tax expense |  |  | (19) |
Amounts reclassified from accumulated other comprehensive income, net |  |  | 60 |
Total other comprehensive loss attributable to Aon shareholders |  |  | 62 |
Ending Balance | $ (2,513) |  | $ (2,513) |

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