Convertible Notes - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) d $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)
Debt Instrument [Line Items]
Principal amount of debt issued	$ 125,000,000	$ 125,000,000
Maturity period for 2014 convertible notes	7 years
Unamortized debt discount and issuance costs	$ (1,155,000)	(1,461,000)
Amortization of debt discount and issuance costs	$ 300,000	300,000	$ 300,000
Convertible debt
Debt Instrument [Line Items]
Initial conversion rate (in percentage)	0.04032
Maximum increase to initial conversion rate (in shares) | shares	0.01207
Convertible debt | 2014 Notes
Debt Instrument [Line Items]
Principal amount of debt issued	$ 125,000,000	125,000,000		$ 125,000,000
Convertible notes, interest rate (in percentage)				1.75%
Debt instrument, convertible, stock price trigger (in dollars per share) | $ / shares	$ 30
Debt instrument, convertible, threshold trading days | d	30
Ownership cap for holder and affiliates (in percentage)	9.985%
Conversion price of notes (in dollars per share) | $ / shares	$ 24.80
Initial conversion, premium over closing price (in percentage)	30.00%
Default provisions, qualifying percentage of ownership (more than)	50.00%
Percentage of voting rights pledged as collateral	65.00%
Interest expense related to 2014 convertible note	$ 2,100,000	$ 2,200,000	$ 2,200,000
Unamortized debt discount and issuance costs	$ (2,146,000)