Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Total revenues	$ 1,285	$ 1,178	$ 2,689	$ 2,435
Employee compensation and benefits	640	585	1,323	1,216
Other operating expenses	211	173	398	334
(Gain)/loss on disposal		(31)	1	(29)
Interest expense	51	49	96	97
Change in estimated acquisition earn-out payables	11	1	7	(2)
Income before income taxes	311	346	738	712
Operating Segments
Segment Reporting Information [Line Items]
Total revenues	1,260	1,164	2,654	2,410
Income before income taxes	360	359	810	740
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	697	646	1,604	1,452
Employee compensation and benefits	385	355	833	755
Other operating expenses	120	110	241	225
(Gain)/loss on disposal	0	(2)	1	(1)
Depreciation and amortization	41	34	85	68
Interest expense	15	19	30	38
Change in estimated acquisition earn-out payables	9	1	3	0
Income before income taxes	127	129	411	367
Operating Segments | Programs
Segment Reporting Information [Line Items]
Total revenues	381	359	709	657
Employee compensation and benefits	109	111	222	219
Other operating expenses	71	70	140	133
(Gain)/loss on disposal	0	(29)	0	(28)
Depreciation and amortization	15	16	31	31
Interest expense	6	7	12	16
Change in estimated acquisition earn-out payables	0	1	2	1
Income before income taxes	180	183	302	285
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	182	159	341	301
Employee compensation and benefits	93	82	179	158
Other operating expenses	27	24	49	44
(Gain)/loss on disposal	0	0	0	0
Depreciation and amortization	4	4	8	8
Interest expense	3	3	6	6
Change in estimated acquisition earn-out payables	2	(1)	2	(3)
Income before income taxes	53	47	97	88
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	25	14	35	25
Income before income taxes	$ (49)	$ (13)	$ (72)	$ (28)