Net income (loss) per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic Net Income (Loss) Per Share
Basic net income (loss)	$ 52,394	$ (12,484)	$ 11,254	$ (199,567)
Weighted average common shares outstanding	212,210,000	205,141,000	211,945,000	200,200,000
Basic net income (loss) per share	$ 0.25	$ (0.06)	$ 0.05	$ (1.00)
Diluted Net Income (Loss) Per Share
Diluted net income (loss)	$ 52,394	$ (12,484)	$ 11,254	$ (199,567)
Weighted average common shares outstanding	212,210,000	205,141,000	211,945,000	200,200,000
Dilutive stock options and RSU’s	2,893,000		3,349,000
Weighted average shares used to compute diluted net income (loss) per share	215,103,000	205,141,000	215,294,000	200,200,000
Diluted net income (loss) per share	$ 0.24	$ (0.06)	$ 0.05	$ (1.00)
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	8,597,202	12,603,888	7,250,307	12,603,888
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	8,079,446	12,085,346	6,732,551	12,085,346
Convertible Senior Notes 0.375 Percent Due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	148,817	149,603	148,817	149,603
Convertible Senior Notes 1.25 Percent Due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	368,939	368,939	368,939	368,939