SEGMENT DISCLOSURES - Summary of Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Revenue	$ 838,014	$ 494,297
Corporate & other:
General & administrative expenses	28,070	18,711
Interest expense	25,156	4,511
Other (expense) income, net	1,966	(9,424)
Income before income tax expense	172,910	104,014
Operating Segments
Segment Reporting Information [Line Items]
Revenue	838,014	494,297
Personnel costs	488,122	293,760
Facility costs	26,920	15,580
Other costs, gains, and losses, net	90,562	51,661
Total segment expense	605,604	361,001
Corporate & other:
Income before income tax expense	232,410	133,296
Operating Segments | Financial Services
Segment Reporting Information [Line Items]
Revenue	713,661	372,630
Personnel costs	411,302	216,615
Facility costs	23,710	12,335
Other costs, gains, and losses, net	75,296	36,525
Total segment expense	510,308	265,475
Corporate & other:
Income before income tax expense	203,353	107,155
Operating Segments | Benefits and Insurance Services
Segment Reporting Information [Line Items]
Revenue	112,976	108,408
Personnel costs	66,665	65,723
Facility costs	3,208	3,182
Other costs, gains, and losses, net	15,158	14,688
Total segment expense	85,031	83,593
Corporate & other:
Income before income tax expense	27,945	24,815
Operating Segments | National Practices
Segment Reporting Information [Line Items]
Revenue	11,377	13,259
Personnel costs	10,155	11,422
Facility costs	2	63
Other costs, gains, and losses, net	108	448
Total segment expense	10,265	11,933
Corporate & other:
Income before income tax expense	1,112	1,326
Corporate And Eliminations
Corporate & other:
Unallocated corporate operating expenses	3,797	15,354
General & administrative expenses	28,070	18,711
Interest expense	25,156	4,511
Other (expense) income, net	$ 2,477	$ (9,294)