Leases - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Lease cost	$ 6,810	$ 6,100	$ 19,426	$ 16,300
Operating lease, weighted-average remaining lease term	7 years 9 months 18 days		7 years 9 months 18 days
Operating lease, weighted-average discount rate	3.70%		3.70%
Current portion of operating lease liabilities	$ 16,700		$ 16,700
Canonsburg Office, New Company Headquarters | Lease Agreement Effective September 14, 2012
Lessee, Lease, Description [Line Items]
Area of real estate property | ft²	186,000		186,000
Period of leased property	183 months		183 months
Base rent, years one through five	$ 4,300		$ 4,300
Base rent, years six through ten	4,500		4,500
Base rent, years eleven through fifteen	$ 4,700		$ 4,700