Loans Receivable and Allowance for Loan Losses - Mortgage Loans Receivable Modified in a Troubled Debt Restructuring (Details) - Mortgage Loans $ in Thousands	12 Months Ended
Dec. 31, 2015 USD ($) loan	Dec. 31, 2014 USD ($) loan	Dec. 31, 2013 USD ($) loan
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	50	56	100
Pre- Modification Recorded Investment	$ 18,927	$ 22,744	$ 43,099
Recorded investment	$ 18,556	$ 21,471	$ 41,008
Residential \| Full documentation interest-only
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	12	21	26
Pre- Modification Recorded Investment	$ 4,620	$ 9,244	$ 6,760
Recorded investment	$ 4,496	$ 8,726	$ 6,730
Residential \| Full documentation amortizing
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	19	4	11
Pre- Modification Recorded Investment	$ 5,063	$ 889	$ 3,753
Recorded investment	$ 4,894	$ 812	$ 3,734
Residential \| Reduced documentation interest-only
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	10	19	37
Pre- Modification Recorded Investment	$ 3,431	$ 6,819	$ 12,199
Recorded investment	$ 3,429	$ 6,774	$ 12,227
Residential \| Reduced documentation amortizing
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	7	5	11
Pre- Modification Recorded Investment	$ 2,911	$ 809	$ 3,404
Recorded investment	$ 2,902	$ 745	$ 3,325
Multi-family
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	0	4	8
Pre- Modification Recorded Investment	$ 0	$ 2,501	$ 6,751
Recorded investment	$ 0	$ 1,981	$ 5,888
Commercial real estate
Information about mortgage loans receivable by segment and class modified in troubled debt restructuring
Number of Loans \| loan	2	3	7
Pre- Modification Recorded Investment	$ 2,902	$ 2,482	$ 10,232
Recorded investment	$ 2,835	$ 2,433	$ 9,104

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_MortgageReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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: