Net income (loss) per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Diluted (in dollars per share)	$ 0.65	$ 0.24	$ 0.14	$ 0.36	$ 1.70	$ 0.05	$ 0.25	$ 0.28	$ 1.39	$ 2.29	$ (1.16)
Net income (numerator):
Net income (loss)	$ 149,573	$ 55,050	$ 33,452	$ 86,520	$ 408,561	$ 11,027	$ 61,110	$ 68,276	$ 324,595	$ 548,974	$ (260,739)
Weighted-average common shares outstanding (denominator):
Weighted-average common shares outstanding—basic (in shares)									232,339	239,096	225,512
Net income (loss) per common share:
Basic (in dollars per share)	$ 0.65	$ 0.24	$ 0.14	$ 0.36	$ 1.71	$ 0.05	$ 0.26	$ 0.29	$ 1.40	$ 2.30	$ (1.16)
Diluted (in shares)									233,172	240,122	225,512
Non-vested restricted stock awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									813	880	0
Outstanding stock option awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									20	122	0
Non-vested performance share awards
Weighted-average common shares outstanding (denominator):
Non-vested restricted stock awards (in shares)									0	24	0