Investment Securities Investment Securities (Summary of Investment in Agency Security) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	$ 69,343	$ 98,670
Debt Securities, Trading and Available-for-sale	71,776	100,442
Total agency MBS, at fair value	70,650	98,887
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	225	198
Debt Securities, Trading and Available-for-sale	204	209
Commercial Mortgage Backed Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	360	352
Debt Securities, Trading and Available-for-sale	348	370
Credit Risk Transfer Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	775	961
Debt Securities, Trading and Available-for-sale	574	976
Fixed Income Securities [Member] \| Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	67,284	96,375
Debt Securities, Trading and Available-for-sale	69,901	98,074
Adjustable-Rate [Member] \| Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	118	160
Debt Securities, Trading and Available-for-sale	120	163
Collateralized Mortgage Obligations [Member] \| Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	406	441
Debt Securities, Trading and Available-for-sale	422	447
Multifamily [Member] \| Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	37	37
Debt Securities, Trading and Available-for-sale	42	39
Interest Only And Principal Only Strip [Member] \| Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	138	146
Debt Securities, Trading and Available-for-sale	165	164
Agency Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Debt Securities, Trading and Available-for-sale, Amortized Cost Basis	$ 67,983	$ 97,159

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) at cost.

No definition available.

+ Details

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

- Definition

Represents total investment in agency securities at fair value.

+ References

No definition available.

+ Details

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

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FixedIncomeSecuritiesMember
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:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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