Leases - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Lessee Lease Description [Line Items]
Operating lease, existence of option to extend	true
Operating lease, option to extend lease terms, description	Operating lease payments include $13.4 million related to options to extend lease terms that are reasonably certain of being exercised
Operating lease payments	$ 93,904,000
Operating leases rental expense		$ 20,114,000	$ 19,126,000	$ 14,444,000
Non-cancellable rental income, 2019		$ 1,600,000
Options to Eextend Lease Terms [Member]
Lessee Lease Description [Line Items]
Operating lease payments	$ 13,400,000
Minimum [Member]
Lessee Lease Description [Line Items]
Operating lease renewal term	1 year
Maximum [Member]
Lessee Lease Description [Line Items]
Operating lease renewal term	5 years