Accumulated Other Comprehensive Income (Loss) [Roll Forward]

No definition available.

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

- Definition

Other Comprehensive Income Reclassification Adjustment For Sale Of Security Reversal Of Prior Period Unrealized Gain And Losses On Realization

+ References

No definition available.

+ Details

Name:	agnc_OtherComprehensiveIncomeReclassificationAdjustmentForSaleOfSecurityReversalOfPriorPeriodUnrealizedGainAndLossesOnRealization
Namespace Prefix:	agnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Accumulated unrealized gain (loss), after tax, on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e681-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580

+ Details

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

- Definition

Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669646-108580

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580

+ Details

Name:	us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

Investment Securities (Summary Of Changes In Accumulated OCI For Available-For-Sale Security) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Unrealized Gains and (Losses), Net	$ 361	$ (193)	$ (145)	$ (621)	$ (205)	$ 90	$ 121	$ 46	$ (598)	$ 52	$ (370)
Agency Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning OCI Balance				$ (345)				$ (397)	(345)	(397)	(66)
Unrealized Gains and (Losses), Net									(677)	(31)	(261)
Reversal of Unrealized (Gains) and Losses, Net on Realization									79	83	(109)
Ending OCI Balance	$ (943)				$ (345)				$ (943)	$ (345)	$ (397)