Acquisitions - Summary of Components of Purchase Consideration Transferred and Preliminary Allocation of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Goodwill	$ 539,803	$ 515,321	$ 477,532
Two Thousand Nineteen Acquisitions [Member]
Assets
Cash and cash equivalents	922
Goodwill	24,857
Receivables, net	4,348
Fixed Assets, net	15
Other intangible assets, net	2,075
Other assets	236
Total assets	32,453
Current liabilities
Current portion of accounts payable, accrued expenses and other liabilities	1,365
Accrued compensation	2,125
Total liabilities	3,490
Net assets acquired	$ 28,963
Two Thousand Eighteen Acquisitions [Member]
Assets
Cash and cash equivalents		1,110
Goodwill		42,188
Receivables, net		50,731
Fixed Assets, net		1,276
Other intangible assets, net		4,677
Other assets		2,894
Total assets		102,876
Current liabilities
Current portion of accounts payable, accrued expenses and other liabilities		15,937
Accrued compensation		26,765
Total liabilities		42,702
Net assets acquired		$ 60,174