Asset Quality - Changes in Outstanding Unpaid Principal Balance, Carrying Amount, and Accretable Yield for PCI Loans (Details) - PCI Loans [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2016
Accretable Yield
Balance at beginning of period	$ 132	$ 131	$ 197
Additions	0	0	(32)
Accretion	(11)	(23)	(44)
Net reclassifications from nonaccretable to accretable	6	28	15
Payments received, net	6	(15)	(4)
Disposals	0	0	(1)
Balance at end of period	121	121	131
Carrying Amount	622	622	738	$ 812	$ 865
Outstanding Unpaid Principal Balance	$ 669	$ 669	$ 803	$ 930	$ 1,002

Certain Loans Acquired in transfer Not Accounted for as Debt Securities, Accretable Yield, Payments Received, Net

No definition available.

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- Definition

Certain Loans Acquired in Transfer Accounted for as Debt Securities, Accretable Yield, Reclassification (to) from Nonaccretable Difference

+ References

No definition available.

+ Details

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- Definition

Purchased credit impaired loans carrying value.

+ References

No definition available.

+ Details

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- Definition

Purchased credit impaired loans outstanding principal balance.

+ References

No definition available.

+ Details

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- Definition

The aggregate excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan for loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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- Definition

Current period recognition due to accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan for loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

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- Definition

Increases in the aggregate excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan due to the acquisition of similar loans accounted for as debt securities during the period. This relates to loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

+ Details

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- Definition

Sale or transfer of a loan during the period resulting in a reduction in the excess of a loan's cash flows expected to be collected over the investor's initial investment. This relates to loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

+ Details

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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- Details

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