Supplementary Balance Sheet Information - Cash, and Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and cash equivalents in domestic accounts	$ 430,819	$ 396,577
Cash and cash equivalents in foreign accounts	62,388	53,780
Total cash and cash equivalents	493,207	450,357
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	494,238	451,370	$ 493,916	$ 425,810
Other long-term assets
Restricted cash included in other long-term assets	$ 1,031	$ 1,013