Debt - Convertible Senior Notes, Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) d $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
If-converted amount exceed principal	$ 1,700,000
Interest expense, debt	500,000	$ 2,100,000
Convertible Senior Notes
Debt Instrument [Line Items]
Face amount				$ 172,500,000.0
Debt issuance cost				5,100,000
Deferred financing costs	$ 428,000			$ 4,200,000
Remaining amortization period of discount on the liability component	1 year 9 months 18 days		2 years	6 years
Adjustments to additional paid in capital	$ 900,000
Stated interest rate	3.25%			6.90%
Number of shares of common stock issued | shares	31.1891
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger, Following Consecutive Business Days	98.00%
Conversion price (in usd per share) | $ / shares	$ 32.06
Number of threshold trading days | d	20
Number of threshold consecutive trading days | d	30
Threshold percentage of stock price trigger	130.00%
Number of threshold consecutive business days	5 days
Repurchase percentage	100.00%
Interest expense, debt		2,600,000
Amortization of debt discount (premium)		1,300,000
Interest expense		$ 1,300,000
Convertible Senior Notes | 3.25% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Stated interest rate	3.25%
Interest expense, debt	$ 1,000,000
Amortization of debt discount (premium)	500,000
Interest expense	$ 500,000