Convertible 4.5% Senior Notes (Details)	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) $ / shares
Convertible debt
Interest rate (as a percent)	4.50%		4.50%
Interest expense	$ 24,000	$ 1,125,000
Convertible Notes
Convertible debt
Interest rate (as a percent)	4.50%		4.50%
Principal amount of debt			$ 2,100,000	$ 100,000,000
Proceeds from issuance of debt				96,600,000
Issuance of debt transaction costs				3,400,000
Debt converted			$ 97,900,000
Shares issued with debt conversion (in shares) | shares			26,160,187
Shares issued with debt conversion adjustment (in shares) | shares			2,784,870
Principal amount of debt for conversion calculations				$ 1,000
Ratio issued upon conversion				238.7775
Initial conversion price (in dollars per share) | $ / shares				$ 4.19