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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Balance | $ 5,532 | $ 6,059 | $ 6,527
Other comprehensive income (loss) before reclassifications | 195 | (791) | (389)
Tax benefit (expense) | 47 | 77 | 23
Other comprehensive income (loss) before reclassifications, net | 242 | (714) | (366)
Amounts reclassified from accumulated other comprehensive income (loss) | 227 | 332 | 128
Tax benefit (expense) | (53) | (107) | (51)
Amounts reclassified from accumulated other comprehensive income (loss), net | 174 | 225 | 77
Total other comprehensive income (loss) attributable to Aon shareholders | 416 | (489) | (289)
Balance | 4,648 | 5,532 | 6,059
Change in Fair Value of Investments |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Balance | (37) | (25) | (17)
Other comprehensive income (loss) before reclassifications | 18 | (25) | (4)
Tax benefit (expense) | (3) | 6 | 1
Other comprehensive income (loss) before reclassifications, net | 15 | (19) | (3)
Amounts reclassified from accumulated other comprehensive income (loss) | (2) | 10 | 11
Tax benefit (expense) | (1) | (3) | (16)
Amounts reclassified from accumulated other comprehensive income (loss), net | (3) | 7 | (5)
Total other comprehensive income (loss) attributable to Aon shareholders | 12 | (12) | (8)
Balance | (25) | (37) | (25)
Foreign Currency Translation Adjustments |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Balance | (1,264) | (771) | (335)
Other comprehensive income (loss) before reclassifications | 397 | (490) | (467)
Tax benefit (expense) | (5) | (3) | 31
Other comprehensive income (loss) before reclassifications, net | 392 | (493) | (436)
Amounts reclassified from accumulated other comprehensive income (loss) | (7) | 0 | 0
Tax benefit (expense) | 0 | 0 | 0
Amounts reclassified from accumulated other comprehensive income (loss), net | (7) | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | 385 | (493) | (436)
Balance | (879) | (1,264) | (771)
Post-retirement Benefit Obligations |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Balance | (2,611) | (2,627) | (2,782)
Other comprehensive income (loss) before reclassifications | (220) | (276) | 82
Tax benefit (expense) | 55 | 74 | (9)
Other comprehensive income (loss) before reclassifications, net | (165) | (202) | 73
Amounts reclassified from accumulated other comprehensive income (loss) | 236 | 322 | 117
Tax benefit (expense) | (52) | (104) | (35)
Amounts reclassified from accumulated other comprehensive income (loss), net | 184 | 218 | 82
Total other comprehensive income (loss) attributable to Aon shareholders | 19 | 16 | 155
Balance | (2,592) | (2,611) | (2,627)
AOCI Including Portion Attributable to Noncontrolling Interest |  |  |
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward] |  |  |
Balance | (3,912) | (3,423) | (3,134)
Balance | $ (3,496) | $ (3,912) | $ (3,423)

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