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Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income | $ 382 | $ 228 | $ 287 | $ 676 | $ 353 | $ 153 | $ 58 | $ 610 | $ 1,573 | $ 1,174 | $ 1,263
Less: Net income attributable to noncontrolling interests | 8 | 6 | 10 | 17 | 8 | 6 | 10 | 16 | 41 | 40 | 37
Net income attributable to Aon shareholders | $ 374 | $ 222 | $ 277 | $ 659 | $ 345 | $ 147 | $ 48 | $ 594 | 1,532 | 1,134 | 1,226
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 3 | 11 | 12
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 14 | (444) | 390
Postretirement benefit obligation |  |  |  |  |  |  |  |  | (141) | 17 | 19
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | (124) | (416) | 421
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | 0 | 0 | 0
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | (4) | 5
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (124) | (412) | 416
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,408 | 722 | 1,642
Aon Ireland |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 1,540 | 0 | 0
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,540 | 0 | 0
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Postretirement benefit obligation |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | (132) | 0 | 0
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (132) | 0 | 0
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,408 | 0 | 0
Aon UK |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 1,540 | 1,155 | 1,208
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,540 | 1,155 | 1,208
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Postretirement benefit obligation |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | (132) | (433) | 434
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (132) | (433) | 434
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 1,408 | 722 | 1,642
Aon Global Holdings Limited |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 210 | 32 | 431
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 210 | 32 | 431
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Postretirement benefit obligation |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | 0 | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | (172) | (417) | 417
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (172) | (417) | 417
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 38 | (385) | 848
Aon Corporation |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 409 | 134 | 247
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 409 | 134 | 247
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 3 | 0 | 3
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Postretirement benefit obligation |  |  |  |  |  |  |  |  | (44) | (2) | (101)
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | (41) | (2) | (98)
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | (131) | (415) | 515
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (172) | (417) | 417
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 237 | (283) | 664
Other Non-Guarantor Subsidiaries |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 2,510 | 2,238 | 2,252
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 41 | 40 | 37
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 2,469 | 2,198 | 2,215
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 0 | 11 | 9
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 6 | (465) | 408
Postretirement benefit obligation |  |  |  |  |  |  |  |  | (97) | 19 | 120
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | (91) | (435) | 537
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | 0 | 0 | 0
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | (4) | 5
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | (91) | (431) | 532
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | 2,378 | 1,767 | 2,747
Consolidating Adjustments |  |  |  |  |  |  |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | (4,636) | (2,385) | (2,875)
Less: Net income attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net income attributable to Aon shareholders |  |  |  |  |  |  |  |  | (4,636) | (2,385) | (2,875)
Other comprehensive income (loss), net of tax: |  |  |  |  |  |  |  |  |  |  |
Change in fair value of financial instruments |  |  |  |  |  |  |  |  | 0 | 0 | 0
Foreign currency translation adjustments |  |  |  |  |  |  |  |  | 8 | 21 | (18)
Postretirement benefit obligation |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) |  |  |  |  |  |  |  |  | 8 | 21 | (18)
Equity in other comprehensive income (loss) of subsidiaries, net of tax |  |  |  |  |  |  |  |  | 567 | 1,265 | (1,366)
Less: Other comprehensive income (loss) attributable to noncontrolling interests |  |  |  |  |  |  |  |  | 0 | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders |  |  |  |  |  |  |  |  | 575 | 1,286 | (1,384)
Comprehensive income attributable to Aon shareholders |  |  |  |  |  |  |  |  | $ (4,061) | $ (1,099) | $ (4,259)

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