ACQUISITIONS - Business Acquisitions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Allocation of purchase price:
Goodwill	$ 1,111,780	$ 1,090,231	$ 860,495
Business acquisitions
ACQUISITIONS
Number of business acquisitions closed	3
Consideration:
Cash due at closing	$ 53,274
Estimated working capital adjustment	(763)
Fair market value of estimated contingent consideration	3,367
Total consideration	55,878
Allocation of purchase price:
Total tangible assets	4,142
Total liabilities assumed	(10,800)
Goodwill	26,545
Total allocated consideration	55,878
Amount of goodwill and intangibles expected to be deductible for tax purposes	$ 32,379
Deductibility period for goodwill and intangible assets acquired in a business acquisition (in years)	15 years
Number of business acquisitions closed during the period which are new subsidiary partner firms | item	1
Revenue from acquired entity in business acquisitions	$ 1,245
Income from acquired entity in business acquisitions	353
Business acquisitions | Customer relationships
Allocation of purchase price:
Finite-lived intangible assets	34,849
Business acquisitions | Management contracts
Allocation of purchase price:
Finite-lived intangible assets	972
Business acquisitions | Other intangibles
Allocation of purchase price:
Finite-lived intangible assets	$ 170