Accumulated Other Comprehensive Income Loss - AOCI Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 18,556
Other comprehensive loss, net of tax	(906)	$ (1,009)
Ending balance	17,702	15,266
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	2,834	1,684
OCI before reclassifications	(922)	(1,015)
Amounts reclassified from AOCI	(3)	(42)
Other comprehensive loss, net of tax	(925)	(1,057)
Ending balance	1,909	627
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	12	9
OCI before reclassifications	12	47
Amounts reclassified from AOCI	(7)	(3)
Other comprehensive loss, net of tax	5	44
Ending balance	17	53
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	43	34
OCI before reclassifications	1	(8)
Amounts reclassified from AOCI	0	0
Other comprehensive loss, net of tax	1	(8)
Ending balance	44	26
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,717)	(1,672)
OCI before reclassifications	0	(1)
Amounts reclassified from AOCI	13	12
Other comprehensive loss, net of tax	13	11
Ending balance	(1,704)	(1,661)
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,170	52
OCI before reclassifications	(909)	(976)
Amounts reclassified from AOCI	3	(33)
Other comprehensive loss, net of tax	(906)	(1,009)
Ending balance	264	(957)
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(2)	(3)
OCI before reclassifications	0	1
Amounts reclassified from AOCI	0	0
Other comprehensive loss, net of tax	0	1
Ending balance	$ (2)	$ (2)