Long-Term Debt - Convertible Senior Notes Due 2032 (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Mar. 20, 2018	Mar. 31, 2012	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 04, 2018	Nov. 01, 2016	Jul. 31, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Principal amount			$ 479,161
Repurchase of convertible debt			60,365	$ 0	$ 138,401
Loss on extinguishment of long-term debt			(1,183)	(397)	(3,540)
Convertible Senior Notes Maturing March 2032
Debt Instrument [Line Items]
Principal amount		$ 200,000	0	$ 60,115
Interest rate (as a percent)		3.25%
Maturity date		Mar. 15, 2032
Repurchased principal amount	$ 59,300					$ 800	$ 125,000	$ 7,600	$ 7,300
Repurchase of convertible debt	59,500
Payments for fees	$ 200
Loss on extinguishment of long-term debt			$ (200)		$ (3,500)