Derivative financial instruments - Table of Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Asset [Abstract]
Gross Amounts of Recognized Assets	$ 13	$ 336
Derivative Asset, Fair Value, Gross Liability	0	0
Amounts of Assets Presented in the Consolidated Balance Sheet	13	336
Derivative Asset, Not Offset, Policy Election Deduction	8	37
Cash Collateral Received or Pledged - Asset, Gross Amount not Offset in the Consolidated Balance Sheet	0	0
Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	5	299
Derivative Liability [Abstract]
Gross amounts of Recognized Liabilities	(93)	(37)
Derivative Liability, Fair Value, Gross Asset	0	0
Amounts of Liabilities Presented in the Consolidated Balance Sheet	(93)	(37)
Derivative Liability, Not Offset, Policy Election Deduction	8	37
Cash Collateral Received or Pledged - Liability, Gross Amount not Offset in the Consolidated Balance Sheet	0	0
Derivative Liability, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	$ 85	$ 0