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Loans and Allowances for Credit Losses, Troubled Debt Restructurings (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructuring, Recorded Investment	$ 171.0	$ 129.0	$ 171.0	$ 129.0	$ 126.0
Troubled Debt Restructuring, Performing in Accordance With Modified Terms	83.0	60.0	83.0	60.0	48.0
Financing Receivable, Modifications, Post-Modification Recorded Investment	31.0	11.0	76.0	53.0
Troubled Debt Restructuring, Charge-offs	4.5	4.4	10.2	4.4
Permanent mortgages guaranteed by US government agencies [Member] | Accruing [Member] | Residential mortgage [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructuring, Recorded Investment	$ 83.0	$ 69.0	$ 83.0	$ 69.0	$ 74.0

X

- Definition

Represents the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructuring and are performing according to modified terms.

+ References

No definition available.

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+ References

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- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
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