Loans and Allowance for Loan Losses - Average Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loan impairment
Impaired Financing Receivable, Related Allowance	$ 75	$ 73
Impaired Financing Receivable, with Related Allowance, Recorded Investment	521	567
Average Recorded Investment	763	922	$ 837
Recognized Interest Income	64	84	15
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	44	776	1,976
Loans receivable sold during the period	638	926	1,526
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	209	298
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	603	645
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	249	326
Commercial
Loan impairment
Impaired Financing Receivable, Related Allowance	62	58
Impaired Financing Receivable, with Related Allowance, Recorded Investment	324	320
Average Recorded Investment	464	566	548
Recognized Interest Income	47	64	5
Allowance for loan losses:
Transfer of loans from held for investment to held for sale		780	1,632
Loans receivable sold during the period		926	1,182
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	126	224
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	397	381
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	143	233
Commercial \| Commercial and industrial
Loan impairment
Impaired Financing Receivable, Related Allowance	61	57
Impaired Financing Receivable, with Related Allowance, Recorded Investment	299	287
Average Recorded Investment	309	463	531
Recognized Interest Income	12	18	4
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	22	93
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	372	348
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	39	102
Commercial \| Commercial mortgage
Loan impairment
Impaired Financing Receivable, Related Allowance	1	1
Impaired Financing Receivable, with Related Allowance, Recorded Investment	25	33
Average Recorded Investment	60	58	17
Recognized Interest Income	27	41	1
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	41	3
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	25	33
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	41	3
Commercial \| Construction
Loan impairment
Average Recorded Investment	95	45	0
Recognized Interest Income	8	5	0
Construction \| Construction loans portfolio [Member]
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	63	128
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	63	128
Consumer
Loan impairment
Impaired Financing Receivable, Related Allowance	13	15
Impaired Financing Receivable, with Related Allowance, Recorded Investment	197	247
Average Recorded Investment	299	356	289
Recognized Interest Income	17	20	10
Allowance for loan losses:
Transfer of loans from held for investment to held for sale		(4)	344
Loans receivable sold during the period		0	344
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	83	74
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	206	264
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	106	93
Consumer \| Residential mortgage
Loan impairment
Impaired Financing Receivable, Related Allowance	12	15
Impaired Financing Receivable, with Related Allowance, Recorded Investment	175	218
Average Recorded Investment	261	304	258
Recognized Interest Income	13	14	8
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	71	59
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	184	234
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	85	69
Consumer \| Home equity and other consumer loans
Loan impairment
Impaired Financing Receivable, Related Allowance	1	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	22	29
Average Recorded Investment	38	52	31
Recognized Interest Income	4	6	$ 2
Allowance for loan losses:
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	12	15
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance	22	30
Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance	21	24
Commercial Portfolio Segment [Member]
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	47
Loans receivable sold during the period	638
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 Related Allowance, Unpaid Principal Balance	0	$ 0
Consumer Portfolio Segment [Member]
Allowance for loan losses:
Transfer of loans from held for investment to held for sale	(3)
Loans receivable sold during the period	$ 0

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The amount of financing receivables reclassified to held for sale during the reporting period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Section 50
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Section 50
-Paragraph 14A
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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