Acquisitions and Dispositions of Businesses - Schedule of Consideration Transferred and Preliminary Value of Intangible Assets (Details) - USD ($) $ in Millions		6 Months Ended	9 Months Ended
	Apr. 25, 2024	Jun. 30, 2025	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Assets acquired:
Goodwill			$ 15,704	$ 15,234
Liabilities assumed:
Payments to acquire businesses, escrow deposits				$ 416
Funds held on behalf of clients			8,391		$ 7,940
2025 Acquisitions
Consideration transferred:
Cash			788
Deferred and contingent consideration			43
Aggregate consideration transferred			831
Assets acquired:
Goodwill			476
Intangible assets			379
Other assets			198
Total assets acquired			1,053
Liabilities assumed:
Total liabilities assumed			222
Net assets acquired			831
Cash and cash equivalents			39
Funds held on behalf of clients			$ 62
NFP Acquisition
Consideration transferred:
Cash	$ 3,200	$ 3,247
Class A ordinary shares issued		5,882
Aggregate consideration transferred	$ 9,100	9,129
Assets acquired:
Goodwill		6,838
Intangible assets		6,775
Other assets		1,362
Total assets acquired		14,975
Liabilities assumed:
Long-term debt		3,422
Deferred tax liabilities		1,013
Other liabilities		1,218
Total liabilities assumed		5,653
Less: Fair value of redeemable noncontrolling interests		(108)
Less: Fair value of nonredeemable noncontrolling interests		(85)
Net assets acquired		9,129
Cash and cash equivalents		294
Funds held on behalf of clients		277
Accounts payable and accrued liabilities		283
Business combination, recognized liability assumed, lease obligation		$ 125