Loans and Allowance for Loan Losses - Average Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loan impairment
Impaired Financing Receivable, Related Allowance	$ 55	$ 75
Impaired Financing Receivable, with Related Allowance, Recorded Investment	373	521
Average Recorded Investment	844	763	$ 922
Recognized Interest Income	53	64	84
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	718	44	776
Loans receivable sold during the period	952	638	926
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	309	209
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	423	603
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	415	249
Commercial
Allowance for loan losses:
Transfer of loans from held for investment to held for sale		47	780
Loans receivable sold during the period		638	926
Commercial \| Commercial and industrial
Loan impairment
Impaired Financing Receivable, Related Allowance	43	61
Impaired Financing Receivable, with Related Allowance, Recorded Investment	186	299
Average Recorded Investment	399	309	463
Recognized Interest Income	11	12	18
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	13	22
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	227	372
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	99	39
Commercial \| Commercial mortgage
Loan impairment
Impaired Financing Receivable, Related Allowance	0	1
Impaired Financing Receivable, with Related Allowance, Recorded Investment	7	25
Average Recorded Investment	119	60	58
Recognized Interest Income	16	27	41
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	168	41
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	7	25
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	168	41
Commercial \| Construction
Loan impairment
Average Recorded Investment	62	95	45
Recognized Interest Income	10	8	5
Consumer
Allowance for loan losses:
Transfer of loans from held for investment to held for sale		(3)	(4)
Loans receivable sold during the period		0	0
Consumer \| Other Consumer Loans [Member]
Loan impairment
Impaired Financing Receivable, Related Allowance	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2	1
Average Recorded Investment	2	1	1
Recognized Interest Income	0	0	0
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0	0
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	2	1
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	0	0
Consumer \| residential mortgage and home equity [Member]
Loan impairment
Impaired Financing Receivable, Related Allowance	12	13
Impaired Financing Receivable, with Related Allowance, Recorded Investment	178	196
Average Recorded Investment	262	298	355
Recognized Interest Income	16	17	20
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	66	83
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	187	205
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	86	106
Commercial Portfolio Segment [Member]
Loan impairment
Impaired Financing Receivable, Related Allowance	43	62
Impaired Financing Receivable, with Related Allowance, Recorded Investment	193	324
Average Recorded Investment	580	464	566
Recognized Interest Income	37	47	64
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	723
Loans receivable sold during the period	952
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	243	126
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	234	397
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	329	143
Commercial Portfolio Segment [Member] \| Construction
Loan impairment
Impaired Financing Receivable, Related Allowance	0	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	0
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	62	63
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	0	0
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	62	63
Consumer Portfolio Segment [Member]
Loan impairment
Impaired Financing Receivable, Related Allowance	12	13
Impaired Financing Receivable, with Related Allowance, Recorded Investment	180	197
Average Recorded Investment	264	299	356
Recognized Interest Income	16	17	$ 20
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	(5)
Loans receivable sold during the period	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	66	83
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	189	206
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	$ 86	$ 106

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