Leases (Details) ft² in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Lease rental expense incurred	$ 17,100	$ 16,900	$ 16,500
Future minimum lease payments	65,113
Future minimum lease payments, Year 1	14,139
Future minimum lease payments, Year 2	11,570
Future minimum lease payments, Year 3	8,785
Future minimum lease payments, Year 4	7,675
Future minimum lease payments, Year 5	7,046
Future minimum lease payments, Thereafter	$ 15,898
Lease Agreement Effective September 14, 2012 | Canonsburg Office, New Company Headquarters
Operating Leased Assets [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
Lease rental expense incurred	$ 4,400	$ 4,400	$ 4,400