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Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic Net Income (Loss) Per Share
Basic net income (loss)									$ 446,906	$ 109,493	$ (313,142)
Weighted average common shares outstanding	215,770,000	215,199,000	214,620,000	214,065,000	213,013,000	212,627,000	212,210,000	211,681,000	214,913,000	212,383,000	204,580,000
Basic net income (loss) per share	$ 0.51	$ 0.60	$ 0.49	$ 0.48	$ 0.32	$ 0.14	$ 0.25	$ (0.19)	$ 2.08	$ 0.52	$ (1.53)
Diluted Net Income (Loss) Per Share
Diluted net income (loss)									$ 446,906	$ 109,493	$ (313,142)
Weighted average common shares outstanding	215,770,000	215,199,000	214,620,000	214,065,000	213,013,000	212,627,000	212,210,000	211,681,000	214,913,000	212,383,000	204,580,000
Dilutive stock options and awards									2,744,000	3,252,000
Weighted average shares used to compute diluted net income (loss) per share	218,542,000	217,791,000	217,483,000	217,061,000	216,042,000	215,964,000	215,103,000	211,681,000	217,657,000	215,635,000	204,580,000
Diluted net income (loss) per share	$ 0.51	$ 0.59	$ 0.48	$ 0.47	$ 0.32	$ 0.14	$ 0.24	$ (0.19)	$ 2.05	$ 0.51	$ (1.53)
Anti-dilutive securities
Potential common shares excluded from diluted net income (loss) per share computation									9,718,828	8,624,931	13,103,527
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net income (loss) per share computation									9,349,889	8,255,992	12,585,213
Convertible Senior Notes 0.375 Percent Due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net income (loss) per share computation											149,375
Convertible Senior Notes 1.25 Percent Due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net income (loss) per share computation									368,939	368,939	368,939