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Credit and Other Debt Agreements - Additional Information (Detail) - USD ($) $ in Millions				3 Months Ended	9 Months Ended
	Aug. 13, 2021	Jul. 29, 2021	May 20, 2021	Sep. 30, 2021	Sep. 30, 2021
Debt Instrument [Line Items]
Loss on extinguishment of debt	$ 16.2			$ 16.2	$ 16.2
Willis Towers Watson PLC [Member]
Debt Instrument [Line Items]
Agreement termination date		Jul. 29, 2021
Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 1,500.0
Weighted average interest rate			3.31%
Realized a cash gain (loss) on hedging transaction			$ (57.8)
Derivative instrument, term			10 years
Fixed Rate of 2.50%, Balloon Due May 20, 2031 [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 650.0
Periodic Payment of Interest			2.50%
Periodic payments of interest and principal, expiry year			2031
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
Notes redemption date		Aug. 13, 2021
Loss on extinguishment of debt	16.2
Redemption premium	6.5
Unamortized discount amount debt issuance and write-off debt acquisition costs	$ 9.7
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member]
Debt Instrument [Line Items]
Periodic Payment of Interest				3.50%	3.50%
Fixed Rate of 3.50%, Balloon Due May 20, 2051 [Member] | Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Notes issued and sold			$ 850.0
Periodic Payment of Interest			3.50%
Periodic payments of interest and principal, expiry year			2051

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