Investment Securities, AFS and HTM Contractual Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in one year or less	$ 2,795
Due after one year through five years	32,399
Due after five years through ten years	317,292
Due after ten years	180,422
Total debt securities	532,908
Investment securities available for sale, amortized cost	3,251,950	$ 4,041,902
Fair Value
Due in one year or less	2,800
Due after one year through five years	32,901
Due after five years through ten years	326,361
Due after ten years	187,098
Total debt securities, AFS Fair Value	549,160
Investment securities available for sale, at fair value	$ 3,262,586	3,946,941
Ratio of Fair Value to Amortized Cost	100.30%
Amortized Cost
Due in one year or less	$ 29,033
Due after one year through five years	82,023
Due after five years through ten years	136,138
Due after ten years	1,172,373
Total debt securities, HTM Amortized Cost	1,419,568
Amortized Cost	2,205,083	2,740,511
Fair Value
Due in one year or less	29,218
Due after one year through five years	83,245
Due after five years through ten years	140,704
Due after ten years	1,235,077
Total debt securities, HTM Fair Value	1,488,245
Debt Securities, Held-to-maturity, Fair Value	$ 2,276,465	2,710,271
Ratio of Fair Value to Amortized Cost	103.20%
FFELP asset backed securities
Amortized Cost
Investment securities available for sale, amortized cost	$ 270,178
Fair Value
Investment securities available for sale, at fair value	263,693
Amortized Cost
Amortized Cost	0
Fair Value
Debt Securities, Held-to-maturity, Fair Value	0
FNMA/FHLMC [Member] \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	131,158	296,296
Fair Value
Investment securities available for sale, at fair value	132,660	295,252
Amortized Cost
Amortized Cost	81,676	92,788
Fair Value
Debt Securities, Held-to-maturity, Fair Value	83,420	91,162
FNMA/FHLMC [Member] \| Commercial mortgage-related securities
Amortized Cost
Investment securities available for sale, amortized cost	19,929
Fair Value
Investment securities available for sale, at fair value	21,728
Amortized Cost
Amortized Cost	0
Fair Value
Debt Securities, Held-to-maturity, Fair Value	0
GNMA [Member] \| Residential Related Securities
Amortized Cost
Investment securities available for sale, amortized cost	982,941	2,169,943
Fair Value
Investment securities available for sale, at fair value	985,139	2,128,531
Amortized Cost
Amortized Cost	269,523	351,606
Fair Value
Debt Securities, Held-to-maturity, Fair Value	270,296	345,035
GNMA [Member] \| Commercial mortgage-related securities
Amortized Cost
Investment securities available for sale, amortized cost	1,314,836	1,273,309
Fair Value
Investment securities available for sale, at fair value	1,310,207	1,220,797
Amortized Cost
Amortized Cost	434,317	505,434
Fair Value
Debt Securities, Held-to-maturity, Fair Value	$ 434,503	$ 490,414

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=asb_FNMAFHLMCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=asb_GNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: