Business Combinations - Additional Information (Details) $ in Millions			1 Months Ended	9 Months Ended
	Oct. 28, 2024 USD ($) Acquisition	Sep. 30, 2024 USD ($)	Oct. 28, 2024 USD ($)	Sep. 30, 2024 USD ($) Acquisition	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition				22
Measurement period adjustment, fiduciary assets				$ 20
Measurement period adjustment, fiduciary liabilities				20
Measurement period adjustment, goodwill				7
Cash paid		$ 4
Measurement period adjustment, non-controlling interest				6
Measurement period adjustment, amortizable intangible assets				11
Measurement period adjustment, other net asstes				(3)
Measurement period adjustment, goodwill				7
Measurement period adjustment, total considration				9
Net assets acquired		187		187
Total consideration		187		187
Goodwill		127		127
Estimated acquisition earn-out payables		137		137	$ 169	$ 249	$ 258	$ 231	$ 252
Maximum future contingency payments acquisitions		460		460
Subsequent Event
Business Acquisition [Line Items]
Number of acquisitions | Acquisition	1
Maximum purchase price for acquisition			$ 74
Other Noncurrent Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables		64		64
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables		73		73
Retail
Business Acquisition [Line Items]
Measurement period adjustment, goodwill				(37)
Wholesale Brokerage
Business Acquisition [Line Items]
Measurement period adjustment, goodwill				$ 6
Purchased Customer Accounts
Business Acquisition [Line Items]
Weighted average life (years)				15 years
Maximum | Subsequent Event
Business Acquisition [Line Items]
Total consideration	$ 50		$ 50
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition				7
Assets and Assumed Certain Liabilities
Business Acquisition [Line Items]
Number of acquisitions | Acquisition				11
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition				4
Other Acquisitions | Maximum
Business Acquisition [Line Items]
Total consideration		$ 50		$ 50