Net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic Net Income (Loss) Per Share
Net income (loss)	$ 55,178	$ (39,582)	$ 9,294	$ (18,359)	$ (36,944)	$ 59,293	$ (36,873)	$ (33,957)	$ 6,531	$ (48,481)	$ (83,147)
Weighted average common shares outstanding	186,269,000	181,387,000	178,676,000	172,070,000	169,924,000	168,592,000	167,914,000	165,357,000	179,601,000	167,947,000	148,403,000
Basic net income (loss) per share	$ 0.30	$ (0.22)	$ 0.05	$ (0.11)	$ (0.22)	$ 0.35	$ (0.22)	$ (0.21)	$ 0.04	$ (0.29)	$ (0.56)
Diluted Net Income (Loss) Per Share
Diluted net income (loss)									$ 6,531	$ (48,481)	$ (83,147)
Weighted average common shares outstanding	186,269,000	181,387,000	178,676,000	172,070,000	169,924,000	168,592,000	167,914,000	165,357,000	179,601,000	167,947,000	148,403,000
Dilutive stock options and RSU’s									7,701,000
Weighted average shares used to compute diluted net income (loss) per share	193,367,000	181,387,000	186,493,000	172,070,000	169,924,000	189,046,000	167,914,000	165,357,000	187,302,000	167,947,000	148,403,000
Diluted net income (loss) per share	$ 0.29	$ (0.22)	$ 0.05	$ (0.11)	$ (0.22)	$ 0.33	$ (0.22)	$ (0.21)	$ 0.03	$ (0.29)	$ (0.56)
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									14,981,092	40,297,462	45,620,469
Net proceeds from issuance of convertible senior and subordinated notes											$ 728,696
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									494,468	15,453,520	20,123,089
4.75% Convertible Senior Notes due 2015
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation										10,353,416	11,006,854
Aggregate principal amount of notes converted											$ 186,000
Common stock issued in exchange of notes (in shares)											21,208,303
Repurchase of notes											$ 117,300
Shares purchased by related party											13,368,883
4.75% Convertible Senior Notes due 2015 | Director
Anti-dilutive securities
Net proceeds from issuance of convertible senior and subordinated notes											$ 500,000
0.375% Convertible Senior Notes due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									7,245,263	7,245,263	7,245,263
1.25% Convertible Senior Notes due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation									7,241,361	7,245,263	7,245,263