Long-Term Debt - Convertible Senior Notes Due 2022 (Details) $ / shares in Units, $ in Thousands		1 Months Ended
	Nov. 01, 2016 USD ($) $ / shares	Mar. 31, 2012 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Mar. 20, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Principal amount			$ 512,400
Unamortized debt discount			33,177
Convertible Senior Notes Maturing May 2022
Debt Instrument [Line Items]
Principal amount	$ 125,000		125,000		$ 125,000
Net proceeds from debt issuance	$ 121,700
Interest rate (as a percent)	4.25%
Frequency of periodic payment	semi-annually
Maturity date	May 01, 2022
Initial conversion ratio	0.0719748
Initial conversion price per share (in dollars per share) | $ / shares	$ 13.89
Redemption price as a percentage of principal amount	100.00%
Minimum percentage in aggregate principal amount	25.00%
Unamortized debt discount	$ 16,900		13,188		13,900
Carrying amount of equity component	$ 11,000
Effective interest rate (as a percent)	7.30%
Convertible Senior Notes Maturing March 2032
Debt Instrument [Line Items]
Principal amount		$ 200,000	809		60,115
Repurchased principal amount	$ 125,000			$ 59,300
Interest rate (as a percent)		3.25%
Maturity date		Mar. 15, 2032
Initial conversion ratio		0.0399752
Initial conversion price per share (in dollars per share) | $ / shares		$ 25.02
Redemption price as a percentage of principal amount		100.00%
Unamortized debt discount		$ 35,400	$ 0		$ 500
Carrying amount of equity component		$ 22,500
Effective interest rate (as a percent)		6.90%