Loans Receivable and Allowance for Loan Losses - Impaired Loans by Segment and Class (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
With an allowance recorded:
Related Allowance	$ (14,813)	$ (19,732)
Without an allowance recorded:
Unpaid Principal Balance	271,192	289,737
Impaired loans	237,518	248,436
Related Allowance	(14,813)	(19,732)
Net Investment	222,705	228,704
Residential \| Full documentation interest-only
With an allowance recorded:
Unpaid Principal Balance	37,454	55,352
Recorded Investment	30,631	46,331
Related Allowance	(4,051)	(3,391)
Net Investment	26,580	42,940
Without an allowance recorded:
Related Allowance	(4,051)	(3,391)
Residential \| Full documentation amortizing
With an allowance recorded:
Unpaid Principal Balance	69,242	43,044
Recorded Investment	63,223	39,994
Related Allowance	(2,534)	(1,425)
Net Investment	60,689	38,569
Without an allowance recorded:
Related Allowance	(2,534)	(1,425)
Residential \| Reduced documentation interest-only
With an allowance recorded:
Unpaid Principal Balance	55,939	90,171
Recorded Investment	46,540	76,960
Related Allowance	(4,253)	(4,661)
Net Investment	42,287	72,299
Without an allowance recorded:
Related Allowance	(4,253)	(4,661)
Residential \| Reduced documentation amortizing
With an allowance recorded:
Unpaid Principal Balance	57,955	19,463
Recorded Investment	52,520	18,117
Related Allowance	(2,310)	(827)
Net Investment	50,210	17,290
Without an allowance recorded:
Related Allowance	(2,310)	(827)
Multi-family
With an allowance recorded:
Unpaid Principal Balance	8,029	34,972
Recorded Investment	7,950	28,109
Related Allowance	(456)	(3,172)
Net Investment	7,494	24,937
Without an allowance recorded:
Unpaid Principal Balance	19,523	16,308
Recorded Investment	16,693	14,502
Net Investment	16,693	14,502
Related Allowance	(456)	(3,172)
Commercial real estate
With an allowance recorded:
Unpaid Principal Balance	6,651	24,991
Recorded Investment	6,723	19,270
Related Allowance	(788)	(2,446)
Net Investment	5,935	16,824
Without an allowance recorded:
Unpaid Principal Balance	11,104	0
Recorded Investment	8,270	0
Net Investment	8,270	0
Related Allowance	(788)	(2,446)
Consumer and Other Loans \| Home equity lines of credit
With an allowance recorded:
Unpaid Principal Balance	5,295	5,436
Recorded Investment	4,968	5,153
Related Allowance	(421)	(3,810)
Net Investment	4,547	1,343
Without an allowance recorded:
Related Allowance	$ (421)	$ (3,810)

Represents the net amount of investment of impaired financing receivables.

No definition available.

Name:	af_ImpairedFinancingReceivableNetInvestment
Namespace Prefix:	af_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the net amount of investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

No definition available.

+ Details

Name:	af_ImpairedFinancingReceivableNetInvestmentWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	af_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the net amount of investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

No definition available.

+ Details

Name:	af_ImpairedFinancingReceivableNetInvestmentWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	af_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impaired Financing Receivable, with No Related Allowance [Abstract]

+ References

No definition available.

+ Details

Name:	af_ImpairedFinancingReceivablewithNoRelatedAllowanceAbstract
Namespace Prefix:	af_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Impaired Financing Receivable, with Related Allowance [Abstract]

+ References

No definition available.

+ Details

Name:	af_ImpairedFinancingReceivablewithRelatedAllowanceAbstract
Namespace Prefix:	af_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5212-111524

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=af_FullDocumentationInterestOnlyLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=af_FullDocumentationAmortizingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=af_ReducedDocumentationInterestOnlyLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=af_ReducedDocumentationAmortizingLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=af_ConsumerAndOtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: