Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Business Acquisition
Additional goodwill recognized		$ 559,903		$ 559,903		$ 557,914	$ 515,321
Business acquisition, aggregate revenue contribution		1,400		3,300
2020 Acquisitions
Business Acquisition
Total consideration transferred				9,800
Payments to acquire businesses, gross				$ 5,900
Business acquisition price paid (Partnership Units) (in shares)				104,653
Business acquisition, contingent non cash consideration, fair value		1,300		$ 1,300
Contingent consideration				2,200
Business acquisition, amount deductible for tax		$ 2,400		2,400
2020 Acquisitions | Restricted Stock Units (RSUs)
Business Acquisition
Shares issued as compensation				$ 400
HVS
Business Acquisition
Total consideration transferred	$ 9,839
Contingent consideration	3,590
Additional goodwill recognized	$ 6,294
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		$ 1,700
2019 Acquisitions | Newmark Holdings, L.P. | Limited Partnership Units
Business Acquisition
Business acquisition price paid (Partnership Units) (in shares)						327,692
Business acquisition, contingent non cash consideration, fair value						$ 2,700