Fair Value Measurements (Details Textuals) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fair Value Measurements (Textuals) [Abstract]
Other real estate owned measured at fair value	$ 2	$ 21
Write down of Other Real Estate Owned		2
Commitment on standby letters of credit	$ 334	$ 353
Fair Value, Option, Quantitative Disclosures [Line Items]
Mortgage derivative interest rate lock commitments closing ratio	86.00%asb_FairValueInputsClosingRatio
Mortgage servicing rights (MSR) weighted average constant prepayment rate	15.00%
Weighted average discount rate	9.60%
Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Weighted average discount rate	30.00%
Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Weighted average discount rate	20.00%

Represents the percentage of loans currently in a lock position which management estimates will ultimately close.

No definition available.

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- Definition

Other real estate owned fair value disclosure.

+ References

No definition available.

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- Definition

Discount rate which is used to value residual cash flows generated by financial assets of a securitization, asset-backed financing arrangement, or similar transfer regardless of when the transfer occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=13989330&loc=d3e107314-111719

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- Definition

Estimated rate of prepayment of principal on financial assets regardless of when the transfer occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 20

-Section 50

-Paragraph 4

-Subparagraph (b)(2)

-URI http://asc.fasb.org/extlink&oid=13989330&loc=d3e107314-111719

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- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

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- Definition

Amount of write-down or reserve recognized in the income statement on real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.FN5)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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