Accounting for Derivative Instruments and Hedging Activities - Effects on accumulated OCL balance attributable to hedge derivatives, net (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Derivative [Line Items]
Balance	$ 2,258	$ 1,968
Reclassified from accumulated OCL to income:
Balance	2,071	2,071
Balance, tax	(5)	(5)
AOCI Including Portion Attributable to Noncontrolling Interest | Interest rate contracts
Derivative [Line Items]
Balance	(31)	(54)
Reclassified from accumulated OCL to income:
Balance	(23)	(23)
Accumulated Gain (Loss), Net, Cash Flow Hedge | Interest rate contracts
Reclassified from accumulated OCL to income:
Due to realization of previously deferred amounts	3	7
Accumulated Net Gain (Loss) from Cash Flow Hedges | Interest rate contracts
Reclassified from accumulated OCL to income:
Mark-to-market of cash flow hedge accounting contracts	$ 5	$ 24