Long-Term Debt - Convertible Senior Notes Due 2032 (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended
	Mar. 20, 2018	Mar. 31, 2012	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Nov. 01, 2016
Debt Instrument [Line Items]
Principal amount			$ 512,400
Repurchase of convertible debt			59,478	$ 0
Loss on extinguishment of long-term debt			(1,105)	$ 0
Unamortized debt discount			33,177
Convertible Senior Notes Maturing March 2032
Debt Instrument [Line Items]
Principal amount		$ 200,000	809		$ 60,115
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)		$ 25.02
Repurchase price as a percentage of principal amount		100.00%
Redemption price as a percentage of principal amount		100.00%
Repurchased principal amount	$ 59,300					$ 125,000
Repurchase of convertible debt	59,500
Payments for fees	200
Loss on extinguishment of long-term debt	$ (200)
Unamortized debt discount		$ 35,400	$ 0		$ 500
Carrying amount of equity component		$ 22,500
Effective interest rate (as a percent)		6.90%