Summary of Significant Accounting Policies - Financial Instruments (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Financial Instruments and Concentration of Credit Risk
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained | item	3
Marketable securities held by entity	$ 0	$ 0
Cash and Cash Equivalents
Cash and cash equivalents	345,058,000	267,107,000	$ 150,337,000	$ 159,964,000
Noncash Investing Activities
Accrued capital expenditures	$ 795,000	$ 482,000