Significant Accounting Policies - Restricted Cash (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted cash
Cash and cash equivalents	$ 276,382,000	$ 155,933,000	$ 150,483,000
Restricted cash included in other current assets	15,000	15,000	9,000
Restricted cash included in other long term assets	500,000		500,000
Total cash, cash equivalents, and restricted cash shown in statement of cash flows	$ 276,897,000	$ 156,448,000	$ 150,992,000	$ 129,758,000