Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	$ 87
Other payable	4
Recorded earn-out payable	5	$ 9
Total consideration	96
Maximum potential earn-out payable	20
Allocation of purchase price:
Cash and equivalents	4
Fiduciary cash	16
Fiduciary receivables	19
Other current assets	2
Goodwill	55
Other assets	4
Total assets acquired	130
Fiduciary liabilities	(29)
Other current liabilities	(4)
Other long-term liabilities	(1)
Total liabilities assumed	(34)
Net assets acquired	96
Various | Other Acquisitions
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash paid	33
Other payable	4
Recorded earn-out payable	5
Total consideration	42
Maximum potential earn-out payable	20
Allocation of purchase price:
Cash and equivalents	4
Fiduciary cash	3
Fiduciary receivables	19
Other current assets	0
Goodwill	22
Other assets	4
Total assets acquired	65
Fiduciary liabilities	(19)
Other current liabilities	(3)
Other long-term liabilities	(1)
Total liabilities assumed	(23)
Net assets acquired	42
Wholesale | NBS Insurance Agency
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	2
Goodwill	33
Other assets	0
Total assets acquired	65
Fiduciary liabilities	(10)
Other current liabilities	(1)
Total liabilities assumed	(11)
Net assets acquired	54
Purchased Customer Accounts and Other Intangibles
Allocation of purchase price:
Purchased customer accounts and other intangibles	30
Purchased Customer Accounts and Other Intangibles | Various | Other Acquisitions
Allocation of purchase price:
Purchased customer accounts and other intangibles	13
Purchased Customer Accounts and Other Intangibles | Wholesale | NBS Insurance Agency
Allocation of purchase price:
Purchased customer accounts and other intangibles	$ 17