Annual report pursuant to Section 13 and 15(d)

Business Segments and Customer Information, Geographic Area (Details)

v3.8.0.1
Business Segments and Customer Information, Geographic Area (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]      
Revenues $ 760,113 $ 691,790 $ 533,982
United States      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Revenues 708,474 638,726 481,466
Other Countries      
Revenues from External Customers and Long-Lived Assets [Line Items]      
Revenues $ 51,639 $ 53,064 $ 52,516