Debt - Convertible Senior Notes, Additional Information (Details)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) d $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Debt conversion, shares issued (in shares) | shares		3,699,043
If-converted amount exceed principal		$ 2,200,000
Interest expense, debt	$ 1,100,000	4,800,000
Convertible Senior Notes | 3.25% Convertible Senior Notes due 2020
Debt Instrument [Line Items]
Face amount					$ 172,500,000.0
Debt issuance cost					5,100,000
Deferred financing costs	$ 546,000	$ 546,000		$ 2,090,000	$ 4,200,000
Remaining amortization period of discount on the liability component		2 years 3 months		3 years	6 years
Adjustments to additional paid in capital		$ 900,000
Stated interest rate	3.25%	3.25%			6.90%
Debt conversion, shares issued (in shares) | 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	$ 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		$ 5,200,000	$ 8,200,000
Amortization of debt discount (premium)		2,700,000	4,100,000
Interest expense		$ 2,500,000	$ 4,100,000