BUSINESS COMBINATIONS - Fair Value of Assets Acquired and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 02, 2021	Dec. 31, 2020
Intangible assets:
Goodwill	$ 787,750	$ 417,700	$ 371,047
Landmark Acquisition
Asset Acquisition [Line Items]
Cash		25,685
Other tangible assets		23,411
Intangible assets:
Total intangible assets		709,240
Total identifiable assets acquired		758,336
Accounts payable, accrued expenses and other liabilities		72,532
Net identifiable assets acquired		685,804
Goodwill		417,665
Net assets acquired		1,103,469
Landmark Acquisition | Management contracts
Intangible assets:
Total intangible assets		425,880
Landmark Acquisition | Client relationships
Intangible assets:
Total intangible assets		197,160
Landmark Acquisition | Trade name
Intangible assets:
Total intangible assets		$ 86,200