Risk Management - Effect of Derivative Contracts on the Income Statement (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Debt fair value adjustments	$ 1,057		$ 1,057		$ 731
Derivatives designated as hedging instruments | Derivatives in fair value hedging relationships | Interest rate contracts | Interest, net
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(loss) recognized in income on derivative and related hedged item	208	$ (81)	336	$ (254)
Derivatives designated as hedging instruments | Derivatives in fair value hedging relationships | Hedged fixed rate debt
Derivative Instruments, Gain (Loss) [Line Items]
Debt fair value adjustments	355		355
Derivatives designated as hedging instruments | Derivatives in fair value hedging relationships | Hedged fixed rate debt | Interest, net
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(loss) recognized in income on derivative and related hedged item	$ (211)	$ 77	$ (349)	$ 245