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Guarantee of Registered Securities - Narrative (Details) | Dec. 31, 2018 | Dec. 03, 2018
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5.00% Senior Notes due September 2020 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 5.00% |
8.205% Junior Subordinated Deferrable Interest Debentures Percent Due 2027 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 8.205% |
6.25% Senior Notes due September 2040 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 6.25% |
4.250% Notes Due 2042 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.25% |
4.45% Senior Notes due May 2043 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.45% |
4.00% Senior Notes due November 2023 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.00% |
2.875% Senior Notes due May 2026 (EUR 500M) |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 2.875% |
3.50% Senior Notes due June 2024 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 3.50% |
4.60% notes due May 2044 |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.60% |
4.500% Senior Notes due December 2028 | Senior Notes |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.50% | 4.50%
4.75% Senior Notes due May 2045 | Senior Notes |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 4.75% |
2.80% Senior Notes due March 2021 | Senior Notes |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 2.80% |
3.875% Senior Notes due December 2025 | Senior Notes |  |
Condensed Financial Statements, Captions [Line Items] |  |
Interest rate on debt | 3.875% |
Aon plc |  |
Condensed Financial Statements, Captions [Line Items] |  |
Subsidiary guarantors percentage owned | 100.00% |

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