Revenue - Disaggregation of revenue on geographic location of customers (Details) - USD ($) $ 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
Revenue
Revenues	$ 496	$ 509	$ 579	$ 557	$ 545	$ 337	$ 375	$ 374	$ 2,141	$ 1,631	$ 1,558
U.S.
Revenue
Revenues									1,371	744	447
Asia
Revenue
Revenues									152	330	424
Canada
Revenue
Revenues									334	295	312
Europe
Revenue
Revenues									118	207	285
Australia
Revenue
Revenues									18	29	30
South America
Revenue
Revenues									134	17	58
Other
Revenue
Revenues									$ 14	$ 9	$ 2