Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) $ in Millions		Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Assets:
Deferred compensation plans		$ 89	$ 74
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.1328	0.1026
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member] | Minimum [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.1161
Contingent consideration [Member] | Fair Value Inputs, Discount Rate [Member] | Maximum [Member]
Liabilities:
Fair value inputs, weighted-average discount rate		0.138
Recurring [Member]
Assets:
Mutual funds / exchange traded funds	[1]	$ 102	$ 7
Derivative financial instruments	[2]	6	26
Liabilities:
Contingent consideration	[3],[4]	31	40
Derivative financial instruments	[2]	1	5
Recurring [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		215	142
Recurring [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	9
Recurring [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1],[6]	8
Recurring [Member] | Level 1 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	102	7
Derivative financial instruments	[2]	0	0
Liabilities:
Contingent consideration	[3],[4]	0	0
Derivative financial instruments	[2]	0	0
Recurring [Member] | Level 1 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		215	142
Recurring [Member] | Level 1 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0
Recurring [Member] | Level 1 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1],[6]	0
Recurring [Member] | Level 2 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	0	0
Derivative financial instruments	[2]	6	26
Liabilities:
Contingent consideration	[3],[4]	0	0
Derivative financial instruments	[2]	1	5
Recurring [Member] | Level 2 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		0	0
Recurring [Member] | Level 2 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0
Recurring [Member] | Level 2 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1],[6]	0
Recurring [Member] | Level 3 [Member]
Assets:
Mutual funds / exchange traded funds	[1]	0	0
Derivative financial instruments	[2]	0	0
Liabilities:
Contingent consideration	[3],[4]	31	40
Derivative financial instruments	[2]	0	0
Recurring [Member] | Level 3 [Member] | Fiduciary Assets [Member]
Assets:
Mutual funds / exchange traded funds		0	$ 0
Recurring [Member] | Level 3 [Member] | Commingled Funds [Member]
Assets:
Deferred compensation plans	[1],[5]	0
Recurring [Member] | Level 3 [Member] | Hedge Funds [Member]
Assets:
Deferred compensation plans	[1],[6]	$ 0

[1] With the exception of the funds included in fiduciary assets, the majority of these balances are held as part of deferred compensation plans with related liabilities in other current liabilities and other non-current liabilities on the consolidated balance sheets.

[2] See Note 10 — Derivative Financial Instruments for further information on our derivative investments

[3] Consideration due to be paid across multiple years until 2027.

[4] Probability weightings are based on our knowledge of the past and planned performance of the acquired entity to which the contingent consideration applies. The fair value weighted-average discount rates used in our material contingent consideration calculations were 13.28% and 10.26% at December 31, 2023 and December 31, 2022, respectively. The range of these discount rates was 11.61% - 13.80% at December 31, 2023. Using different probability weightings and discount rates could result in an increase or decrease of the contingent consideration payable.

[5] Consists of the Towers Watson Global Equity Focus Fund, for which redemptions can occur on any business day, and require a minimum of one business day’s notice.

[6] Consists of the Towers Watson Alternative Credit Fund, for which the redemption period is generally quarterly, however requires a 50-day notice.