Summary of Significant Accounting Policies - Financial Instruments (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 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	$ 0
Cash and Cash Equivalents
Cash and cash equivalents	194,851,000	194,851,000	159,964,000	$ 196,000,000	$ 212,283,000
Noncash Investing Activities
Accrued capital expenditures		120,000	356,000
Aggregate principal amount of Convertible	3,022,000	3,022,000	$ 96,965,000
Convertible Notes
Noncash Investing Activities
Aggregate principal amount of Convertible	$ 96,900,000	$ 96,900,000
Shares issued with debt conversion (in shares) | shares	25,882,421