Earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	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
Earnings per common share
Net (loss) income	$ (950)	$ (289)	$ (40)	$ 15	$ 55	$ 2	$ 12	$ (11)	$ (1,264)	$ 58	$ 68
Weighted average number of common shares outstanding - basic (in shares)									58.1	41.2	36.4
Weighted dilutive effect of equity awards										0.1	0.1
Weighted Average Number of Shares Outstanding, Diluted, Total									58.1	41.3	36.5
(Loss) earnings per common share:
Basic (In dollars per share)	$ (16.10)	$ (4.90)	$ (0.70)	$ 0.27	$ 1.00	$ 0.05	$ 0.33	$ (0.31)	$ (21.76)	$ 1.41	$ 1.87
Diluted (In dollars per share)	$ (16.10)	$ (4.90)	$ (0.70)	$ 0.26	$ 0.98	$ 0.05	$ 0.32	$ (0.31)	$ (21.76)	$ 1.40	$ 1.86