Supplementary Balance Sheet Information - Cash and Cash Equivalent Balances (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Supplementary Balance Sheet Information
Cash and cash equivalents in domestic accounts	$ 400,506	$ 375,122
Cash and cash equivalents in foreign accounts	64,234	50,232
Cash and cash equivalents	$ 464,740	$ 425,354	$ 1,932,111	$ 389,328