Leases - Additional Information (Detail) ft² in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) ft²	Mar. 31, 2018 USD ($)
Lessee, Lease, Description [Line Items]
Lease cost	$ 6,082	$ 4,900
Operating lease, weighted-average remaining lease term	8 years
Operating lease, weighted-average discount rate	3.30%
Current portion of operating lease liabilities	$ 15,300
Canonsburg Office, New Company Headquarters | Lease Agreement Effective September 14, 2012
Lessee, Lease, Description [Line Items]
Area of real estate property | ft²	186
Period of leased property	183 months
Base rent, years one through five	$ 4,300
Base rent, years six through ten	4,500
Base rent, years eleven through fifteen	$ 4,700