Goodwill and intangible assets (Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Gross	$ 642	$ 283
Accumulated amortization	(67)	(30)
Net	575	253
Amortization expense	40	31	$ 3
Amortization expense, 2016	44
Amortization expense, 2017	44
Amortization expense, 2018	44
Amortization expense, 2019	44
Amortization expense, 2020	44
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	410	271
Accumulated amortization	(46)	(18)
Net	364	253
Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	11	12
Accumulated amortization	(11)	(12)
Net	0	0
Long Term Supply Contacts [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	221	0
Accumulated amortization	(10)	0
Net	211	0
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Net	$ 2	$ 2