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Shareholders' Equity (Details 2) - USD ($) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest | $ 5,385 | $ 5,826 | $ 5,155 | $ 4,648 | $ 5,532
Adoption of new accounting guidance |  |  |  | 492 | $ 49
Other comprehensive income before reclassifications, net | 277 |  |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income | 38 |  |  |  |
Tax expense | (9) |  |  |  |
Amounts reclassified from accumulated other comprehensive income, net | 29 |  |  |  |
Total other comprehensive income attributable to Aon shareholders | 306 | $ 162 |  |  |
Total |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest | (3,191) |  |  | (3,497) |
Change in Fair Value of Financial Instruments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest | (12) |  |  | (26) |
Other comprehensive income before reclassifications, net | 11 |  |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income | 4 |  |  |  |
Tax expense | (1) |  |  |  |
Amounts reclassified from accumulated other comprehensive income, net | 3 |  |  |  |
Total other comprehensive income attributable to Aon shareholders | 14 |  |  |  |
Foreign Currency Translation Adjustments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest | (635) |  |  | (879) |
Other comprehensive income before reclassifications, net | 244 |  |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income | 0 |  |  |  |
Tax expense | 0 |  |  |  |
Amounts reclassified from accumulated other comprehensive income, net | 0 |  |  |  |
Total other comprehensive income attributable to Aon shareholders | 244 |  |  |  |
Post-Retirement Benefit Obligation |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest | (2,544) |  |  | (2,592) |
Other comprehensive income before reclassifications, net | 22 |  |  |  |
Amounts reclassified from accumulated other comprehensive loss: |  |  |  |  |
Amounts reclassified from accumulated other comprehensive income | 34 |  |  |  |
Tax expense | (8) |  |  |  |
Amounts reclassified from accumulated other comprehensive income, net | 26 |  |  |  |
Total other comprehensive income attributable to Aon shareholders | $ 48 |  |  |  |
As Reported | Total |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest |  |  |  | (3,496) |
As Reported | Change in Fair Value of Financial Instruments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest |  |  |  | (25) |
As Reported | Foreign Currency Translation Adjustments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest |  |  |  | (879) |
As Reported | Post-Retirement Benefit Obligation |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Stockholders' equity, including portion attributable to noncontrolling interest |  |  |  | (2,592) |
Accounting Standards Update 2016-01 | Total |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Adoption of new accounting guidance |  |  |  | (1) |
Accounting Standards Update 2016-01 | Change in Fair Value of Financial Instruments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Adoption of new accounting guidance |  |  |  | (1) |
Accounting Standards Update 2016-01 | Foreign Currency Translation Adjustments |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Adoption of new accounting guidance |  |  |  | 0 |
Accounting Standards Update 2016-01 | Post-Retirement Benefit Obligation |  |  |  |  |
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] |  |  |  |  |
Adoption of new accounting guidance |  |  |  | $ 0 |

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