Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total revenues	$ 4,805	$ 4,257	$ 3,573
Employee compensation and benefits	2,406	2,187	1,817
Other operating expenses	710	650	597
(Gain)/loss on disposal	(31)	(143)	(5)
Interest	193	190	141
Change in estimated acquisition earn-out payables	2	21	(39)
Income before income taxes	1,303	1,146	876
Operating Segments
Segment Reporting Information [Line Items]
Total revenues	4,745	4,222	3,568
Income before income taxes	1,380	1,214	878
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	2,729	2,508	2,157
Employee compensation and benefits	1,462	1,336	1,126
Other operating expenses	449	421	377
(Gain)/loss on disposal	(3)	(3)	(8)
Depreciation and amortization	140	131	115
Interest	71	85	96
Change in estimated acquisition earn-out payables	8	1	(26)
Income before income taxes	602	537	477
Operating Segments | Programs
Segment Reporting Information [Line Items]
Total revenues	1,400	1,173	958
Employee compensation and benefits	450	425	376
Other operating expenses	290	247	223
(Gain)/loss on disposal	(28)	(141)	1
Depreciation and amortization	62	55	53
Interest	30	36	33
Change in estimated acquisition earn-out payables	(7)	0	(11)
Income before income taxes	603	551	283
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	616	541	453
Employee compensation and benefits	322	284	239
Other operating expenses	92	85	70
(Gain)/loss on disposal	0	0	3
Depreciation and amortization	15	14	12
Interest	11	12	13
Change in estimated acquisition earn-out payables	1	20	(2)
Income before income taxes	175	126	118
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	60	35	5
Income before income taxes	$ (77)	$ (68)	$ (2)