Long-Term Debt - Convertible Senior Notes Due 2023 (Details) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended		6 Months Ended
	Mar. 20, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Mar. 31, 2012 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	May 04, 2018 USD ($)
Debt Instrument [Line Items]
Principal amount				$ 459,224		$ 459,224
Repayment of term loan						33,692	$ 61,936
Unamortized debt discount				25,732		25,732
Interest expense				8,045	$ 8,041	15,941	16,340
Convertible Senior Notes Maturing September 2023
Debt Instrument [Line Items]
Principal amount	$ 125,000			125,000		125,000		$ 125,000
Net proceeds from debt issuance	$ 121,000
Interest rate (as a percent)	4.125%
Frequency of periodic payment	semi-annually
Maturity date	Sep. 15, 2023
Initial conversion ratio	0.1056133
Initial conversion price per share (in dollars per share) | $ / shares	$ 9.47
Redemption price as a percentage of principal amount	100.00%
Minimum percentage in aggregate principal amount	25.00%
Unamortized debt discount	$ 20,100			16,183		16,183		17,800
Carrying amount of equity component	$ 15,900
Effective interest rate (as a percent)	7.80%
Interest expense				2,100	$ 1,900	4,200	$ 2,200
Convertible Senior Notes Maturing March 2032 (Redeemed May 2018)
Debt Instrument [Line Items]
Principal amount			$ 200,000
Repurchased principal amount	$ 59,300								$ 800
Interest rate (as a percent)			3.25%
Maturity date			Mar. 15, 2032
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Principal amount				$ 0		$ 0		$ 33,693
Repayment of term loan		$ 61,000