Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition
Additional goodwill recognized		$ 559,935		$ 559,935		$ 557,914	$ 515,321
Business acquisition, aggregate revenue contribution		1,900		5,200
HVS
Business Acquisition
Total consideration transferred	$ 9,839			9,800
Payments to acquire businesses, gross				$ 5,900
Business acquisition, equity interest issued (in shares)				104,653
Business acquisition, contingent non cash consideration, fair value		1,300		$ 1,300
Contingent consideration	3,590			2,200
Additional goodwill recognized	$ 6,294	6,300		6,300
Business acquisition, amount deductible for tax		$ 2,400		2,400
HVS | Restricted Stock Units (RSUs)
Business Acquisition
Shares issued as compensation				$ 400
2019 Acquisitions
Business Acquisition
Total consideration transferred						56,893
Contingent consideration cash						15,300
Contingent consideration						18,067
Additional goodwill recognized						43,804
Business acquisition, amount deductible for tax						$ 29,700
Business acquisition, aggregate revenue contribution			$ 1,700		$ 3,400
2019 Acquisitions | Newmark Holdings, L.P. | Limited Partnership Units
Business Acquisition
Business acquisition, equity interest issued (in shares)						327,692
Business acquisition, contingent non cash consideration, fair value						$ 2,700