Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Calculations for basic and diluted earnings per common share
Net income	$ 46,672us-gaap_NetIncomeLoss	$ 45,199us-gaap_NetIncomeLoss
Preferred stock dividends	(1,228)	(1,244)
Net income available to common equity	45,444	43,955
Common shareholder dividends	(15,166)us-gaap_DividendsCommonStock	(14,488)us-gaap_DividendsCommonStock
Dividends on unvested share-based payment awards	(114)asb_UnvestedShareBasedPaymentAwards	(151)asb_UnvestedShareBasedPaymentAwards
Undistributed earnings	30,164us-gaap_UndistributedEarnings	29,316us-gaap_UndistributedEarnings
Undistributed earnings allocated to common shareholders	29,886	29,126
Undistributed earnings allocated to unvested share-based payment awards	278	190
Undistributed earnings	30,164asb_UndistributedEarningsAllocations	29,316asb_UndistributedEarningsAllocations
Basic
Distributed earnings to common shareholders	15,166	14,488
Undistributed earnings allocated to common shareholders	29,886asb_UndistributedEarningsBasic	29,126asb_UndistributedEarningsBasic
Total common shareholders earnings, basic	45,052asb_CommonShareholdersEarningsBasic	43,614asb_CommonShareholdersEarningsBasic
Diluted
Distributed earnings to common shareholders	15,166	14,488
Undistributed earnings allocated to common shareholders	29,886us-gaap_UndistributedEarningsDiluted	29,126us-gaap_UndistributedEarningsDiluted
Total common shareholders earnings, diluted	$ 45,052asb_CommonShareholdersEarningsDiluted	$ 43,614asb_CommonShareholdersEarningsDiluted
Weighted average common shares outstanding (in shares)	150,070,000	161,467,000
Effect of dilutive common stock awards (in shares)	1,094,000	721,000
Diluted weighted average common shares outstanding (in shares)	151,164,000	162,188,000
Basic earnings per common share (in usd per share)	$ 0.30us-gaap_EarningsPerShareBasic	$ 0.27us-gaap_EarningsPerShareBasic
Diluted earnings per common share (in usd per share)	$ 0.30us-gaap_EarningsPerShareDiluted	$ 0.27us-gaap_EarningsPerShareDiluted
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share	2,000,000	3,000,000

Common Shareholders Earnings Basic.

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Common Shareholders Earnings Diluted.

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Distributed earnings to common shareholders basic.

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Distributed earnings to common shareholders diluted.

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Undistributed earnings allocated to common shareholders

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Undistributed Earnings Allocated To Unvested Share-Based Payment Awards

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Undistributed earnings allocated to common shareholders and unvested share based payment awards.

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Unvested Share based Payment Awards.

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK).

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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