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Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | $ 4,219 | $ 5,140
Other comprehensive income (loss) before reclassifications, net | 146 |
Amounts reclassified from accumulated other comprehensive income (loss) |  |
Amounts reclassified from accumulated other comprehensive income (loss) | 31 |
Tax expense | (8) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 23 |
Total other comprehensive income attributable to Aon shareholders | 169 | 306
Ending Balance | 4,862 | $ 5,385
Total |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | (3,909) |
Amounts reclassified from accumulated other comprehensive income (loss) |  |
Ending Balance | (3,740) |
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 | 4 |
Amounts reclassified from accumulated other comprehensive income (loss) |  |
Amounts reclassified from accumulated other comprehensive income (loss) | 5 |
Tax expense | (2) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 3 |
Total other comprehensive income attributable to Aon shareholders | 7 |
Ending Balance | (8) |
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 | 131 |
Amounts reclassified from accumulated other comprehensive income (loss) |  |
Amounts reclassified from accumulated other comprehensive income (loss) | 0 |
Tax expense | 0 |
Amounts reclassified from accumulated other comprehensive income (loss), net | 0 |
Total other comprehensive income attributable to Aon shareholders | 131 |
Ending Balance | (1,188) |
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 | 11 |
Amounts reclassified from accumulated other comprehensive income (loss) |  |
Amounts reclassified from accumulated other comprehensive income (loss) | 26 |
Tax expense | (6) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 20 |
Total other comprehensive income attributable to Aon shareholders | 31 |
Ending Balance | $ (2,544) |

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