Summary of Significant Accounting Policies - Financial Instruments (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 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	267,107,000	159,964,000	$ 245,026,000	$ 278,109,000	$ 142,261,000
Noncash Investing Activities
Accrued capital expenditures	$ 482,000	$ 356,000