Long-Term Debt - Convertible Senior Notes Due 2032 (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 01, 2016	Jul. 31, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Principal amount		$ 520,329
Loss on early extinguishment of long-term debt		(397)	$ (3,540)	$ 0
Unamortized debt discount		14,406	19,094
Convertible Senior Notes Maturing March 2032 [Member]
Debt Instrument [Line Items]
Principal amount	$ 200,000	60,115	60,115
Interest rate (as a percent)	3.25%
Frequency of periodic payment	semi-annually
Maturity date	Mar. 15, 2032
Initial conversion ratio	0.0399752
Initial conversion price per share (in dollars per share)	$ 25.02
Redemption price as a percentage of principal amount	100.00%
Repurchase price as a percentage of principal amount	100.00%
Repurchased principal amount					$ 125,000	$ 7,600	$ 7,300
Loss on early extinguishment of long-term debt			(3,500)
Unamortized debt discount	$ 35,400	$ 530	$ 2,600
Carrying amount of equity component	$ 22,500
Expected life used to estimate fair value	6 years
Effective interest rate (as a percent)	6.90%