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Credit and Other Debt Agreements - Additional Information (Detail) - USD ($) |  |  | 3 Months Ended |
--- | --- | --- | --- | ---
Jul. 29, 2021 | May 20, 2021 | Sep. 30, 2021 | Jun. 30, 2021
Subsequent Event [Member] | Willis Towers Watson PLC [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Agreement termination date | Jul. 29, 2021 |  |  |
Private Placement [Member] | Unsecured Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes issued and sold |  | $ 1,500,000,000.0 |  |
Weighted average interest rate |  | 3.31% |  |
Realized a cash gain (loss) on hedging transaction |  | $ (57,800,000) |  |
Derivative instrument, term |  | 10 years |  |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Periodic Payment of Interest |  |  |  | 2.50%
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Unsecured Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Amount payable to redeem the notes, percent of the principal amount following transaction termination event |  | 101.00% |  |
Number of days to occur reinsurance transaction termination event for note redemption |  | 30 days |  |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Unsecured Senior Notes [Member] | Scenario, Forecast [Member] | Minimum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Extinguishment of debt after tax loss |  |  | $ 12,000,000.0 |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Unsecured Senior Notes [Member] | Scenario, Forecast [Member] | Maximum [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Extinguishment of debt after tax loss |  |  | $ 14,000,000.0 |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] | Unsecured Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes issued and sold |  | $ 650,000,000.0 |  |
Periodic Payment of Interest |  | 2.50% |  |
Periodic payments of interest and principal, expiry year |  | 2031 |  |
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Private Placement [Member] | Unsecured Senior Notes [Member] | Subsequent Event [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes redemption date | Aug. 13, 2021 |  |  |
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Periodic Payment of Interest |  |  |  | 3.50%
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Private Placement [Member] | Unsecured Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes issued and sold |  | $ 850,000,000.0 |  |
Periodic Payment of Interest |  | 3.50% |  |
Periodic payments of interest and principal, expiry year |  | 2051 |  |

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