Segment Reporting Selected Segment Income Statement and Balance Sheet Data (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Income Statement Data Abstract
Net interest income	$ 167,813us-gaap_InterestIncomeExpenseNet	$ 164,973us-gaap_InterestIncomeExpenseNet
Noninterest income	80,076us-gaap_NoninterestIncome	73,521us-gaap_NoninterestIncome
Total revenue	247,889us-gaap_Revenues	238,494us-gaap_Revenues
Credit provision	4,500	5,000
Noninterest expense	174,255us-gaap_NoninterestExpense	167,658us-gaap_NoninterestExpense
Income (loss) before income taxes	69,134	65,836
Income tax expense (benefit)	22,462us-gaap_IncomeTaxExpenseBenefit	20,637us-gaap_IncomeTaxExpenseBenefit
Net income	46,672us-gaap_NetIncomeLoss	45,199us-gaap_NetIncomeLoss
Return on average allocated capital (ROTICE)	10.20%asb_ReturnOnAverageAllocatedCapital	9.40%asb_ReturnOnAverageAllocatedCapital
Segment Balance Sheet Data
Average earning assets	24,148,026asb_AverageEarningAssets	21,892,503asb_AverageEarningAssets
Average loans	17,815,115	16,164,617
Average deposits	19,055,196asb_AverageDeposits	16,990,272asb_AverageDeposits
Average allocated capital (TICE)	1,804,217asb_AverageAllocatedCapital	1,899,535asb_AverageAllocatedCapital
Corporate and Commercial Specialty [Member]
Segment Income Statement Data Abstract
Net interest income	75,691	74,065
Noninterest income	12,613	11,854
Total revenue	88,304	85,919
Credit provision	9,526	13,032
Noninterest expense	34,460	35,721
Income (loss) before income taxes	44,318	37,166
Income tax expense (benefit)	15,368	13,008
Net income	28,950	24,158
Return on average allocated capital (ROTICE)	12.50%	11.00%
Segment Balance Sheet Data
Average earning assets	9,218,844	8,850,984
Average loans	9,210,992	8,843,070
Average deposits	5,421,541	5,241,027
Average allocated capital (TICE)	942,764	886,927
Community, Consumer, and Business [Member]
Segment Income Statement Data Abstract
Net interest income	86,357	72,251
Noninterest income	65,514	55,829
Total revenue	151,871	128,080
Credit provision	7,071	4,948
Noninterest expense	118,543	111,199
Income (loss) before income taxes	26,257	11,933
Income tax expense (benefit)	9,190	4,176
Net income	17,067	7,757
Return on average allocated capital (ROTICE)	10.70%	5.90%
Segment Balance Sheet Data
Average earning assets	8,525,399	7,230,394
Average loans	8,525,399	7,230,394
Average deposits	10,522,618	9,539,887
Average allocated capital (TICE)	647,375	535,752
Risk Management and Shared Services [Member]
Segment Income Statement Data Abstract
Net interest income	5,765	18,657
Noninterest income	1,949	5,838
Total revenue	7,714	24,495
Credit provision	(12,097)	(12,980)
Noninterest expense	21,252	20,738
Income (loss) before income taxes	(1,441)	16,737
Income tax expense (benefit)	(2,096)	3,453
Net income	655	13,284
Return on average allocated capital (ROTICE)	(1.10%)	10.20%
Segment Balance Sheet Data
Average earning assets	6,403,783	5,811,125
Average loans	78,724	91,153
Average deposits	3,111,037	2,209,358
Average allocated capital (TICE)	$ 214,078	$ 476,856

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Average deposits

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Average earning assets

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Return on average allocated tier 1 common equity

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This is the result of the average balance during the period of total loans managed or securitized less securitized loans and loans that are in the process of being securitized.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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The net amount of operating interest income (expense).

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Total aggregate amount of all noninterest expense.

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The total amount of noninterest income which may be derived from: (1) fees and commissions; (2) premiums earned; (3) insurance policy charges; (4) the sale or disposal of assets; and (5) other sources not otherwise specified.

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Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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