INTANGIBLE ASSETS AND GOODWILL - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 455,206		$ 455,206		$ 455,206
Advanced Plan Administration
Finite-Lived Intangible Assets [Line Items]
Goodwill	88,900		88,900
Supplemental Benefits Administration
Finite-Lived Intangible Assets [Line Items]
Goodwill					190,200
Value Based Analytics
Finite-Lived Intangible Assets [Line Items]
Goodwill	138,200		138,200
Tradenames, Customer Relationships and Technology
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	5,900	$ 5,900	17,600	$ 17,600
Capitalized software development costs
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	200	$ 100	900	$ 200
Technology Enabled Solutions
Finite-Lived Intangible Assets [Line Items]
Goodwill	417,300		417,300		417,300
Advisory Services
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 37,900		$ 37,900		$ 37,900