Net income per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic Net Income Per Share
Basic net income	$ 105,318	$ 52,394	$ 207,630	$ 11,254
Weighted average common shares outstanding	214,620,000	212,210,000	214,342,000	211,945,000
Basic net income per share	$ 0.49	$ 0.25	$ 0.97	$ 0.05
Diluted Net Income Per Share
Diluted net income	$ 105,318	$ 52,394	$ 207,630	$ 11,254
Weighted average common shares outstanding	214,620,000	212,210,000	214,342,000	211,945,000
Dilutive stock options and awards	2,863,000	2,893,000	2,932,000	3,349,000
Weighted average shares used to compute diluted net income per share	217,483,000	215,103,000	217,274,000	215,294,000
Diluted net income per share	$ 0.48	$ 0.24	$ 0.96	$ 0.05
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	9,534,715	8,597,202	9,590,880	7,250,307
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	9,165,776	8,079,446	9,221,941	6,732,551
Convertible Senior Notes 0.375 Percent Due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation		148,817		148,817
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