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Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic Net Income (Loss) Per Share
Net income (loss)	$ (149,629)	$ 36,054	$ (12,484)	$ (187,083)	$ 8,873	$ 36,877	$ 34,425	$ 24,047	$ (313,142)	$ 104,222	$ 6,531
Weighted average common shares outstanding	211,125,000	206,796,000	205,141,000	195,260,000	188,598,000	188,029,000	187,682,000	187,184,000	204,580,000	187,873,000	179,601,000
Basic net income (loss) per share	$ (0.71)	$ 0.17	$ (0.06)	$ (0.96)	$ 0.05	$ 0.20	$ 0.18	$ 0.13	$ (1.53)	$ 0.55	$ 0.04
Diluted Net Income (Loss) Per Share
Diluted net income (loss)									$ (313,142)	$ 104,222	$ 6,531
Weighted average common shares outstanding	211,125,000	206,796,000	205,141,000	195,260,000	188,598,000	188,029,000	187,682,000	187,184,000	204,580,000	187,873,000	179,601,000
Dilutive stock options and RSU’s										6,252,000	7,701,000
Weighted average shares used to compute diluted net income (loss) per share	211,125,000	212,610,000	205,141,000	195,260,000	195,187,000	194,265,000	193,015,000	192,625,000	204,580,000	194,125,000	187,302,000
Diluted net income (loss) per share	$ (0.71)	$ 0.17	$ (0.06)	$ (0.96)	$ 0.05	$ 0.19	$ 0.18	$ 0.12	$ (1.53)	$ 0.54	$ 0.03
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									13,103,527	17,278,857	14,981,092
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									12,585,213	2,792,424	494,468
Convertible Senior Notes 0.375 Percent Due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									149,375	7,245,149	7,245,263
Convertible Senior Notes 1.25 Percent Due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									368,939	7,241,284	7,241,361