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Shareholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | $ 3,449 | $ 4,219
Other comprehensive income (loss) before reclassifications, net | (403) | 146
Amounts reclassified from accumulated other comprehensive income |  |
Amounts reclassified from accumulated other comprehensive income | 35 | 31
Tax expense | (8) | (8)
Amounts reclassified from accumulated other comprehensive income, net (3) | 27 | 23
Total other comprehensive income (loss) attributable to Aon shareholders | (376) | 169
Ending Balance | 3,260 | 4,862
Total |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | (4,033) | (3,909)
Amounts reclassified from accumulated other comprehensive income |  |
Ending Balance | (4,409) | (3,740)
Change in Fair Value of Financial Instruments |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | (12) | (15)
Other comprehensive income (loss) before reclassifications, net | (9) | 4
Amounts reclassified from accumulated other comprehensive income |  |
Amounts reclassified from accumulated other comprehensive income | 5 | 5
Tax expense | (1) | (2)
Amounts reclassified from accumulated other comprehensive income, net (3) | 4 | 3
Total other comprehensive income (loss) attributable to Aon shareholders | (5) | 7
Ending Balance | (17) | (8)
Foreign Currency Translation Adjustments |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | (1,305) | (1,319)
Other comprehensive income (loss) before reclassifications, net | (395) | 131
Amounts reclassified from accumulated other comprehensive income |  |
Amounts reclassified from accumulated other comprehensive income | 0 | 0
Tax expense | 0 | 0
Amounts reclassified from accumulated other comprehensive income, net (3) | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | (395) | 131
Ending Balance | (1,700) | (1,188)
Postretirement Benefit Obligation |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |
Beginning Balance | (2,716) | (2,575)
Other comprehensive income (loss) before reclassifications, net | 1 | 11
Amounts reclassified from accumulated other comprehensive income |  |
Amounts reclassified from accumulated other comprehensive income | 30 | 26
Tax expense | (7) | (6)
Amounts reclassified from accumulated other comprehensive income, net (3) | 23 | 20
Total other comprehensive income (loss) attributable to Aon shareholders | 24 | 31
Ending Balance | $ (2,692) | $ (2,544)

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