Pledged Assets (Securities Pledged as Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Offsetting Assets and Liabilities [Line Items]
Retained Interests in Consolidated VIE's Pledged as Collateral Under Repo	$ 194	$ 245
Security Owned and Pledged as Collateral, Fair Value	43,145	48,130
Agency Securities Pledged As Collateral Amortized Cost	42,201	48,152
Agency Securities Pledged As Collateral Accrued Interest	118	135
Maturity Less than 30 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	18,604	20,053
Agency Securities Pledged As Collateral Amortized Cost	18,204	20,075
Agency Securities Pledged As Collateral Accrued Interest	51	57
Maturity 31 To 59 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	7,404	8,311
Agency Securities Pledged As Collateral Amortized Cost	7,258	8,340
Agency Securities Pledged As Collateral Accrued Interest	20	23
Maturity 60 To 90 Days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	2,078	7,534
Agency Securities Pledged As Collateral Amortized Cost	2,026	7,525
Agency Securities Pledged As Collateral Accrued Interest	6	21
Maturity over 90 days [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	15,014	12,207
Agency Securities Pledged As Collateral Amortized Cost	14,668	12,187
Agency Securities Pledged As Collateral Accrued Interest	41	34
Maturity Overnight [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	45	25
Agency Securities Pledged As Collateral Amortized Cost	45	25
Agency Securities Pledged As Collateral Accrued Interest	0	0
Agency Securities [Member]
Offsetting Assets and Liabilities [Line Items]
Security Owned and Pledged as Collateral, Fair Value	43,100	48,105
Agency Securities Pledged As Collateral Amortized Cost	42,156	48,127
Agency Securities Pledged As Collateral Accrued Interest	$ 118	$ 135

Item represents accrued interest on agency securities pledged as collateral.

No definition available.

Name:	agnc_AgencySecuritiesPledgedAsCollateralAccruedInterest
Namespace Prefix:	agnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Item represents agency securities pledged as collateral at amortized cost.

+ References

No definition available.

+ Details

Name:	agnc_AgencySecuritiesPledgedAsCollateralAmortizedCost
Namespace Prefix:	agnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

[Line Items] for Offsetting Assets and Liabilities [Table]

+ References

No definition available.

+ Details

Name:	agnc_OffsettingAssetsAndLiabilitiesLineItems
Namespace Prefix:	agnc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Retained Interests in Consolidated VIE's Pledged as Collateral Under Repurchase Agreements

+ References

No definition available.

+ Details

Name:	agnc_RetainedInterestsinConsolidatedVIEsPledgedasCollateralUnderRepurchaseAgreements
Namespace Prefix:	agnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

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

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityUpTo30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=agnc_Maturity31To59DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=agnc_Maturity60To90DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOver90DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOvernightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AgencySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: