Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Total revenues	$ 1,404	$ 1,258
Employee compensation and benefits	683	631
Other operating expenses	186	161
(Gain)/loss on disposal	2	2
Interest expense	46	48
Change in estimated acquisition earn-out payables	(4)	(2)
Income before income taxes	427	364
Operating Segments
Segment Reporting Information [Line Items]
Total revenues	1,394	1,246
Income before income taxes	450	380
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	907	806
Employee compensation and benefits	447	400
Other operating expenses	122	115
(Gain)/loss on disposal	2	1
Depreciation and amortization	43	34
Interest expense	15	19
Change in estimated acquisition earn-out payables	(6)	(1)
Income before income taxes	284	238
Operating Segments | Programs
Segment Reporting Information [Line Items]
Total revenues	328	298
Employee compensation and benefits	113	109
Other operating expenses	69	63
(Gain)/loss on disposal	0	1
Depreciation and amortization	15	15
Interest expense	7	8
Change in estimated acquisition earn-out payables	2	1
Income before income taxes	122	101
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	159	142
Employee compensation and benefits	86	77
Other operating expenses	22	19
(Gain)/loss on disposal	0	0
Depreciation and amortization	5	4
Interest expense	2	3
Change in estimated acquisition earn-out payables	0	(2)
Income before income taxes	44	41
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	10	12
Income before income taxes	$ (23)	$ (16)