Net income (loss) per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic Net Income (Loss) Per Share
Net income (loss)	$ 36,877	$ (39,582)	$ 95,349	$ (48,645)
Weighted average common shares outstanding	188,029,000	181,387,000	187,632,000	177,378,000
Basic net income (loss) per share	$ 0.20	$ (0.22)	$ 0.51	$ (0.27)
Diluted Net Income (Loss) Per Share
Diluted net income (loss)	$ 36,877	$ (39,582)	$ 95,349	$ (48,645)
Weighted average common shares outstanding	188,029,000	181,387,000	187,632,000	177,378,000
Dilutive stock options and RSU’s	6,236,000		6,122,000
Weighted average shares used to compute diluted net income (loss) per share	194,265,000	181,387,000	193,754,000	177,378,000
Diluted net income (loss) per share	$ 0.19	$ (0.22)	$ 0.49	$ (0.27)
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	17,605,587	27,011,919	17,433,642	27,011,919
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	3,119,097	12,520,799	2,947,152	12,520,799
4.75% Convertible Senior Notes due 2015
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation		632		632
Interest rate of debt (as a percent)	4.75%		4.75%
0.375% Convertible Senior Notes due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	7,245,206	7,245,263	7,245,206	7,245,263
Interest rate of debt (as a percent)	0.375%	0.375%	0.375%	0.375%
1.25% Convertible Senior Notes due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	7,241,284	7,245,225	7,241,284	7,245,225
Interest rate of debt (as a percent)	1.25%	1.25%	1.25%	1.25%