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Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Jan. 01, 2018
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |  |
Beginning Balance | $ 4,219 | $ 4,648 | $ 5,581 |
Adoption of new accounting guidance |  |  |  | $ 492
Beginning Balance, adjusted |  |  |  | 5,140
Other comprehensive income (loss) before reclassifications | (282) | (576) | 195 |
Tax benefit (expense) | 58 | 43 | 47 |
Other comprehensive income (loss) before reclassifications, net | (224) | (533) | 242 |
Amounts reclassified from accumulated other comprehensive income (loss) | 123 | 157 | 227 |
Tax benefit (expense) | (23) | (36) | (53) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 100 | 121 | 174 |
Total other comprehensive income (loss) attributable to Aon shareholders | (124) | (412) | 416 |
Ending Balance | 3,449 | 4,219 | 4,648 |
Change in Fair Value of Investments |  |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |  |
Beginning Balance | (15) | (25) | (37) |
Adoption of new accounting guidance |  |  |  | (1)
Beginning Balance, adjusted |  |  |  | (26)
Other comprehensive income (loss) before reclassifications | (10) | (15) | 18 |
Tax benefit (expense) | 1 | 18 | (3) |
Other comprehensive income (loss) before reclassifications, net | (9) | 3 | 15 |
Amounts reclassified from accumulated other comprehensive income (loss) | 14 | 11 | (2) |
Tax benefit (expense) | (2) | (3) | (1) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 12 | 8 | (3) |
Total other comprehensive income (loss) attributable to Aon shareholders | 3 | 11 | 12 |
Ending Balance | (12) | (15) | (25) |
Foreign Currency Translation Adjustments |  |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |  |
Beginning Balance | (1,319) | (879) | (1,264) |
Adoption of new accounting guidance |  |  |  | 0
Beginning Balance, adjusted |  |  |  | (879)
Other comprehensive income (loss) before reclassifications | 15 | (437) | 397 |
Tax benefit (expense) | (1) | (3) | (5) |
Other comprehensive income (loss) before reclassifications, net | 14 | (440) | 392 |
Amounts reclassified from accumulated other comprehensive income (loss) | 0 | 0 | (7) |
Tax benefit (expense) | 0 | 0 | 0 |
Amounts reclassified from accumulated other comprehensive income (loss), net | 0 | 0 | (7) |
Total other comprehensive income (loss) attributable to Aon shareholders | 14 | (440) | 385 |
Ending Balance | (1,305) | (1,319) | (879) |
Post-retirement Benefit Obligations |  |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |  |
Beginning Balance | (2,575) | (2,592) | (2,611) |
Adoption of new accounting guidance |  |  |  | 0
Beginning Balance, adjusted |  |  |  | (2,592)
Other comprehensive income (loss) before reclassifications | (287) | (124) | (220) |
Tax benefit (expense) | 58 | 28 | 55 |
Other comprehensive income (loss) before reclassifications, net | (229) | (96) | (165) |
Amounts reclassified from accumulated other comprehensive income (loss) | 109 | 146 | 236 |
Tax benefit (expense) | (21) | (33) | (52) |
Amounts reclassified from accumulated other comprehensive income (loss), net | 88 | 113 | 184 |
Total other comprehensive income (loss) attributable to Aon shareholders | (141) | 17 | 19 |
Ending Balance | (2,716) | (2,575) | (2,592) |
AOCI attributable to parent |  |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |  |
Beginning Balance | (3,909) | (3,496) | (3,912) |
Adoption of new accounting guidance |  |  |  | (1)
Beginning Balance, adjusted |  |  |  | $ (3,497)
Ending Balance | $ (4,033) | $ (3,909) | $ (3,496) |

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