Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		$ 15,025
Other comprehensive income before reclassification, net of income taxes		(80)	$ (96)
Amounts reclassified from accumulated other comprehensive income, net of income taxes		54	(1)
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate	$ 141		(141)
Other amounts reclassified from AOCI, net of income taxes		(13)
Total other comprehensive income (loss), net of tax		(39)	(238)	$ (136)
Ending Balance	15,025	15,596	15,025
Unrealized Gains (Losses) on Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(311)	(185)
Other comprehensive income before reclassification, net of income taxes		(62)	(69)
Amounts reclassified from accumulated other comprehensive income, net of income taxes		0	(1)
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate			(56)
Other amounts reclassified from AOCI, net of income taxes		0
Total other comprehensive income (loss), net of tax		(62)	(126)
Ending Balance	(311)	(373)	(311)	(185)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(86)	(14)
Other comprehensive income before reclassification, net of income taxes		7	(48)
Amounts reclassified from accumulated other comprehensive income, net of income taxes		29	(10)
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate			(14)
Other amounts reclassified from AOCI, net of income taxes		0
Total other comprehensive income (loss), net of tax		36	(72)
Ending Balance	(86)	(50)	(86)	(14)
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		9	(3)
Other comprehensive income before reclassification, net of income taxes		(10)	12
Amounts reclassified from accumulated other comprehensive income, net of income taxes		0	1
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate			(1)
Other amounts reclassified from AOCI, net of income taxes		(13)
Total other comprehensive income (loss), net of tax		(23)	12
Ending Balance	9	(14)	9	(3)
Net Pension and Postretirement Benefit Costs [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(391)	(339)
Other comprehensive income before reclassification, net of income taxes		(15)	9
Amounts reclassified from accumulated other comprehensive income, net of income taxes		25	9
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate			(70)
Other amounts reclassified from AOCI, net of income taxes		0
Total other comprehensive income (loss), net of tax		10	(52)
Ending Balance	(391)	(381)	(391)	(339)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(779)	(541)	(405)
Total other comprehensive income (loss), net of tax		(39)	(238)	(136)
Ending Balance	$ (779)	$ (818)	$ (779)	$ (541)