Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	$ 183
Other payable	10
Recorded earn-out payable	17	$ 19
Total consideration	210
Maximum potential earn-out payable	48
Allocation of purchase price:
Cash and equivalents	5
Fiduciary cash	17
Fiduciary receivables	22
Other current assets	5
Goodwill	132
Other assets	4
Total assets acquired	246
Fiduciary liabilities	(32)
Other current liabilities	(3)
Other long-term liabilities	(1)
Total liabilities assumed	(36)
Net assets acquired	210
Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	61
NBS Insurance Agency | Wholesale
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	54
Other payable	0
Recorded earn-out payable	0
Total consideration	54
Maximum potential earn-out payable	0
Allocation of purchase price:
Cash and equivalents	0
Fiduciary cash	13
Fiduciary receivables	0
Other current assets	4
Goodwill	30
Other assets	0
Total assets acquired	64
Fiduciary liabilities	(10)
Other current liabilities	0
Total liabilities assumed	(10)
Net assets acquired	54
NBS Insurance Agency | Wholesale | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	17
Tim Parkman, Inc. | Wholesale
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	69
Other payable	6
Recorded earn-out payable	4
Total consideration	79
Maximum potential earn-out payable	23
Allocation of purchase price:
Cash and equivalents	0
Fiduciary cash	0
Fiduciary receivables	0
Other current assets	0
Goodwill	60
Other assets	0
Total assets acquired	79
Fiduciary liabilities	0
Other current liabilities	0
Other long-term liabilities	0
Total liabilities assumed	0
Net assets acquired	79
Tim Parkman, Inc. | Wholesale | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	19
Other Acquisitions | Various
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	60
Other payable	4
Recorded earn-out payable	13
Total consideration	77
Maximum potential earn-out payable	25
Allocation of purchase price:
Cash and equivalents	5
Fiduciary cash	4
Fiduciary receivables	22
Other current assets	1
Goodwill	42
Other assets	4
Total assets acquired	103
Fiduciary liabilities	(22)
Other current liabilities	(3)
Other long-term liabilities	(1)
Total liabilities assumed	(26)
Net assets acquired	77
Other Acquisitions | Various | Purchased Customer Accounts and Other
Allocation of purchase price:
Intangible assets	$ 25