Supplementary Balance Sheet Information - Cash and Cash Equivalent Balances (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Supplementary Balance Sheet Information
Cash and cash equivalents in domestic accounts	$ 372,201	$ 375,122
Cash and cash equivalents in foreign accounts	55,388	50,232
Cash and cash equivalents	$ 427,589	$ 425,354	$ 465,319	$ 389,328