Long-Term Debt - Carrying Value, Convertible Notes (Details) - Convertible Senior Notes 1.00 percent due 2020 - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt
Principal balance	$ 287,500	$ 287,500
Unamortized discount and capitalized debt issuance costs	(35,527)	(46,432)
Net carrying amount of Convertible Notes	$ 251,973	$ 241,068
Conversion price (in dollars per share)	$ 52.35
Adjusted conversion price (in dollars per share)	$ 73.29
Common Share Warrants
Long-Term Debt
Number of shares that may be purchased under warrant	5.5
Warrant strike price (in dollars per share)	$ 73.29
Call Option
Long-Term Debt
Number of shares that may be acquired under call option	5.5
Call option strike price (in dollars per share)	$ 52.35