Leases - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee, Lease, Description [Line Items]
Lease cost	$ 26,261	$ 21,300	$ 18,400
Operating lease, weighted average remaining lease term	7 years 8 months 12 days
Operating lease, weighted average discount rate, percent	3.70%
Current portion of operating lease liabilities	$ 18,200
Operating office lease liabilities that have not yet commenced	$ 16,300
Lessee, operating lease, lease not yet commenced, term of contract	7 years 2 months 12 days
Canonsburg Office, New Company Headquarters | Lease Agreement Effective September 14, 2012
Lessee, Lease, Description [Line Items]
Area of real estate property | ft²	186,000
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