Net income (loss) per common share - Calculation of net income per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income (numerator):
Net income	$ 408,561	$ 11,027	$ 61,110	$ 68,276	$ (18,421)	$ 9,485	$ (71,432)	$ (180,371)	$ 548,974	$ (260,739)	$ (2,209,936)
Weighted-average common shares outstanding (denominator):
Weighted-average common shares outstanding—basic (in shares)									239,096	225,512	199,158
Weighted-average common shares outstanding—diluted (in shares)									240,122	225,512	199,158
Net income (loss) per common share:
Basic (in dollars per share)									$ 2.30	$ (1.16)	$ (11.10)
Diluted (in dollars per share)									$ 2.29	$ (1.16)	$ (11.10)
Non-vested restricted stock awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									880	0	0
Outstanding stock option awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									122	0	0
Non-vested performance share awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									24	0	0