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Guarantee of Registered Securities - Condensed Consolidating Statement of Comprehensive Income (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2020 | Mar. 31, 2019
Condensed Financial Statements, Captions [Line Items] |  |
Net income | $ 791 | $ 676
Less: Net income attributable to noncontrolling interests | 19 | 17
Net income attributable to Aon shareholders | 772 | 659
Change in fair value of financial instruments | (5) | 7
Foreign currency translation adjustments | (397) | 133
Postretirement benefit obligation | 24 | 31
Total other comprehensive income (loss) | (378) | 171
Equity in other comprehensive income (loss) of subsidiaries, net of tax | 0 | 0
Less: Other comprehensive loss attributable to noncontrolling interests | (2) | 2
Total other comprehensive income (loss) attributable to Aon shareholders | (376) | 169
Comprehensive income (loss) attributable to Aon shareholders | 396 | 828
Aon Ireland |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | 773 | 0
Less: Net income attributable to noncontrolling interests | 0 | 0
Net income attributable to Aon shareholders | 773 | 0
Change in fair value of financial instruments | 0 | 0
Foreign currency translation adjustments | 0 | 0
Postretirement benefit obligation | 0 | 0
Total other comprehensive income (loss) | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax | (377) | 0
Less: Other comprehensive loss attributable to noncontrolling interests | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | (377) | 0
Comprehensive income (loss) attributable to Aon shareholders | 396 | 0
Aon UK |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | 773 | 661
Less: Net income attributable to noncontrolling interests | 0 | 0
Net income attributable to Aon shareholders | 773 | 661
Change in fair value of financial instruments | 0 | 0
Foreign currency translation adjustments | 0 | 0
Postretirement benefit obligation | 0 | 0
Total other comprehensive income (loss) | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax | (377) | 167
Less: Other comprehensive loss attributable to noncontrolling interests | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | (377) | 167
Comprehensive income (loss) attributable to Aon shareholders | 396 | 828
Aon Global Holdings Limited |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | 605 | 600
Less: Net income attributable to noncontrolling interests | 0 | 0
Net income attributable to Aon shareholders | 605 | 600
Change in fair value of financial instruments | 0 | 0
Foreign currency translation adjustments | 0 | 0
Postretirement benefit obligation | 0 | 0
Total other comprehensive income (loss) | 0 | 0
Equity in other comprehensive income (loss) of subsidiaries, net of tax | (370) | 139
Less: Other comprehensive loss attributable to noncontrolling interests | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | (370) | 139
Comprehensive income (loss) attributable to Aon shareholders | 235 | 739
Aon Corporation |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | 604 | 513
Less: Net income attributable to noncontrolling interests | 0 | 0
Net income attributable to Aon shareholders | 604 | 513
Change in fair value of financial instruments | (2) | 2
Foreign currency translation adjustments | 0 | 0
Postretirement benefit obligation | 13 | 22
Total other comprehensive income (loss) | 11 | 24
Equity in other comprehensive income (loss) of subsidiaries, net of tax | (381) | 115
Less: Other comprehensive loss attributable to noncontrolling interests | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | (370) | 139
Comprehensive income (loss) attributable to Aon shareholders | 234 | 652
Other Non-Guarantor Subsidiaries |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | 1,045 | 929
Less: Net income attributable to noncontrolling interests | 19 | 17
Net income attributable to Aon shareholders | 1,026 | 912
Change in fair value of financial instruments | (3) | 5
Foreign currency translation adjustments | (398) | 131
Postretirement benefit obligation | 11 | 9
Total other comprehensive income (loss) | (390) | 145
Equity in other comprehensive income (loss) of subsidiaries, net of tax | 0 | 0
Less: Other comprehensive loss attributable to noncontrolling interests | (2) | 2
Total other comprehensive income (loss) attributable to Aon shareholders | (388) | 143
Comprehensive income (loss) attributable to Aon shareholders | 638 | 1,055
Consolidation Adjustments |  |
Condensed Financial Statements, Captions [Line Items] |  |
Net income | (3,009) | (2,027)
Less: Net income attributable to noncontrolling interests | 0 | 0
Net income attributable to Aon shareholders | (3,009) | (2,027)
Change in fair value of financial instruments | 0 | 0
Foreign currency translation adjustments | 1 | 2
Postretirement benefit obligation | 0 | 0
Total other comprehensive income (loss) | 1 | 2
Equity in other comprehensive income (loss) of subsidiaries, net of tax | 1,505 | (421)
Less: Other comprehensive loss attributable to noncontrolling interests | 0 | 0
Total other comprehensive income (loss) attributable to Aon shareholders | 1,506 | (419)
Comprehensive income (loss) attributable to Aon shareholders | $ (1,503) | $ (2,446)

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