Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		$ 15,240
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)	$ 141	141
Total other comprehensive income (loss), net of tax		(238)	$ (136)	$ (49)
Ending Balance	15,025	15,025	15,240
Unrealized Gains (Losses) on Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(185)	(58)
Other comprehensive income before reclassification, net of income taxes		(69)	(127)
Amounts reclassified from accumulated other comprehensive income, net of income taxes (a)		(1)	0
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)		(56)
Total other comprehensive income (loss), net of tax		(126)	(127)
Ending Balance	(311)	(311)	(185)	(58)
Unrealized Gains (Losses) on Derivative Financial Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(14)	20
Other comprehensive income before reclassification, net of income taxes		(48)	17
Amounts reclassified from accumulated other comprehensive income, net of income taxes (a)		(10)	(51)
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)		(14)
Total other comprehensive income (loss), net of tax		(72)	(34)
Ending Balance	(86)	(86)	(14)	20
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(3)	(2)
Other comprehensive income before reclassification, net of income taxes		12	(1)
Amounts reclassified from accumulated other comprehensive income, net of income taxes (a)		1	0
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)		(1)
Total other comprehensive income (loss), net of tax		12	(1)
Ending Balance	9	9	(3)	(2)
Net Pension and Postretirement Benefit Costs [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(339)	(365)
Other comprehensive income before reclassification, net of income taxes		9	5
Amounts reclassified from accumulated other comprehensive income, net of income taxes (a)		9	21
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)		(70)
Total other comprehensive income (loss), net of tax		(52)	26
Ending Balance	(391)	(391)	(339)	(365)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(541)	(405)	(356)
Other comprehensive income before reclassification, net of income taxes		(96)	(106)
Amounts reclassified from accumulated other comprehensive income, net of income taxes (a)		(1)	(30)
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate (b)		(141)
Total other comprehensive income (loss), net of tax		(238)	(136)	(49)
Ending Balance	$ (779)	$ (779)	$ (541)	$ (405)