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Guarantee of Registered Securities - Narrative (Details) - subsidiary | Sep. 30, 2019 | May 02, 2019 | Dec. 31, 2018 | Dec. 03, 2018
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Condensed Financial Statements, Captions [Line Items] |  |  |  |
Number of subsidiaries |  |  | 2 |
Aon plc |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Parent company's percentage ownership of guarantors | 100.00% |  |  |
5.00% Senior notes due September 2020 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 5.00% |  |  |
8.205% Junior subordinated deferrable interest debentures due January 2027 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 8.205% |  |  |
6.25% Senior notes due September 2040 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 6.25% |  |  |
4.250% Senior notes due 2042 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.25% |  |  |
4.45% notes due 2043 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.45% |  |  |
4.00% notes due 2023 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.00% |  |  |
2.875% notes due 2026 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 2.875% |  |  |
3.50% Notes due June 2024 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 3.50% |  |  |
4.60% notes due May 2044 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.60% |  |  |
4.75% Notes Due May 2045 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.75% |  |  |
4.500% Senior Notes Due December 2028 | Senior Notes |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 4.50% |  |  | 4.50%
3.75% Senior Notes Due May 2, 2029 | Senior Notes |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 3.75% | 3.75% |  |
2.80% Senior Notes Due 2021 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 2.80% |  |  |
3.875% due in December 2025 |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |
Debt interest rate percentage (as a percent) | 3.875% |  |  |

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