Significant Accounting Policies - Restricted Cash (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted cash
Cash and cash equivalents	$ 165,867,000	$ 155,933,000	$ 147,099,000
Total cash, cash equivalents, and restricted cash shown in statement of cash flows	166,375,000	156,448,000	147,599,000	$ 129,758,000
Other long-term assets
Restricted cash
Restricted cash	500,000		$ 500,000
Other current assets
Restricted cash
Restricted cash	$ 8,000	$ 15,000