Supplementary Balance Sheet Information - Cash and Cash Equivalent Balances (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Supplementary Balance Sheet Information
Cash and cash equivalents in domestic accounts	$ 353,440	$ 375,122
Cash and cash equivalents in foreign accounts	60,182	50,232
Cash and cash equivalents	$ 413,622	$ 425,354	$ 446,883	$ 389,328