Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Contingent consideration	$ 17,496	$ 32,065
Additional goodwill recognized	559,214	557,914	$ 515,321
Business acquisition, aggregate revenue contribution	1,900
2020 Acquisitions
Business Acquisition [Line Items]
Total consideration transferred	$ 9,800
Business acquisition price paid (Partnership Units) (in shares)	104,653
Business acquisition, contingent non cash consideration, fair value	$ 1,300
Additional goodwill recognized	3,600
Business acquisition, amount deductible for tax	2,400
Payments to Acquire Businesses, Gross	$ 2,200
2019 Acquisitions
Business Acquisition [Line Items]
Total consideration transferred		56,893
Contingent consideration		15,300
Additional goodwill recognized		43,804
Business acquisition, amount deductible for tax		29,700
Business acquisition, aggregate revenue contribution		$ 18,400
2019 Acquisitions | Newmark Holdings, L.P. | Limited Partnership Units
Business Acquisition [Line Items]
Business acquisition price paid (Partnership Units) (in shares)		327,692
Business acquisition, contingent non cash consideration, fair value		$ 2,700