Long-Term Debt - Convertible Senior Notes Due 2023 (Details) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Mar. 20, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2012 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 04, 2018 USD ($)	Nov. 01, 2016 USD ($)	Jul. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)
Debt Instrument [Line Items]
Principal amount					$ 479,161
Repayment of term loan					63,807	$ 194,758	$ 62,742
Unamortized debt discount					(28,802)	(14,406)
Interest expense					32,617	38,274	$ 45,110
Convertible Senior Notes Maturing September 2023
Debt Instrument [Line Items]
Principal amount	$ 125,000				125,000	0
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)				(17,759)
Carrying amount of equity component	$ 15,900
Effective interest rate (as a percent)	7.80%
Interest expense					6,400
Convertible Senior Notes Maturing March 2032
Debt Instrument [Line Items]
Principal amount				$ 200,000	0	60,115
Repurchased principal amount	$ 59,300							$ 800	$ 125,000	$ 7,600	$ 7,300
Interest rate (as a percent)				3.25%
Maturity date				Mar. 15, 2032
Term Loan Maturing June 2020
Debt Instrument [Line Items]
Principal amount		$ 100,000			$ 33,693	$ 97,500
Repayment of term loan			$ 61,000
Interest rate (as a percent)					6.77%
Frequency of periodic payment		quarterly
Unamortized debt discount					$ 0