Summary of Significant Accounting Policies (Policy)	12 Months Ended
Summary of Significant Accounting Policies (Policy)	Dec. 31, 2011
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Basis of Presentation and Consolidation	Basis of Presentation and Consolidation Our consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States ("GAAP"). Our consolidated financial statements include the accounts of our wholly-owned subsidiary, American Capital Agency TRS, LLC, and variable interest entities for which the Company is the primary beneficiary. Significant intercompany accounts and transactions have been eliminated.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and revenues and expenses during the period reported. Actual results could differ from those estimates.
Earnings Per Share	Earnings per Share Basic earnings per share ("EPS") is computed by dividing net income by the weighted average number of common shares outstanding during the period. Diluted EPS assumes the conversion, exercise or issuance of all potential common stock equivalents unless the effect is to reduce a loss or increase the income per share.
Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Accounting Standards Codification ("ASC") Topic 220, Comprehensive Income ("ASC 220") divides comprehensive income into net income and other comprehensive income (loss) ("OCI"), which includes unrealized gains and losses on securities classified as available-for-sale and unrealized gains and losses on derivative financial instruments that are designated and qualify for cash flow hedge accounting under ASC Topic 815, Derivatives and Hedging ("ASC 815"). See Derivatives and Hedging Instruments below and Note 5 regarding our discontinuation of cash flow hedge accounting for interest rate swaps during the third quarter of 2011.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist of unrestricted demand deposits and highly liquid investments with original maturities of three months or less. Cash and cash equivalents are carried at cost, which approximates fair value.
Restricted Cash	Restricted Cash Restricted cash includes cash pledged as collateral for clearing and executing trades, repurchase agreements, interest rate swaps and other derivative instruments. Restricted cash is carried at cost, which approximates fair value.
Investment Securities	Investment Securities ASC Topic 320, Investments—Debt and Equity Securities (“ASC 320”), requires that at the time of purchase, we designate a security as held-to-maturity, available-for-sale or trading, depending on our ability and intent to hold such security to maturity. Securities classified as trading and available-for-sale are reported at fair value, while securities classified as held-to-maturity are reported at amortized cost. We may, from time to time, sell any of our investment securities as part of our overall management of our investment portfolio. Accordingly, we typically designate our investment securities as available-for-sale. All securities classified as available-for-sale are reported at fair value, with unrealized gains and losses reported in OCI of stockholders’ equity. Upon the sale of a security, we determine the cost of the security and the amount of unrealized gains or losses to reclassify out of accumulated OCI into earnings based on the specific identification method. Interest-only securities and inverse interest-only securities (collectively referred to as “interest-only securities”) represent our right to receive a specified proportion of the contractual interest flows of specific agency CMO securities. Principal-only securities represent our right to receive the contractual principal flows of specific agency CMO securities. Interest-only and principal-only securities are measured at fair value through earnings in gain (loss) on derivative instruments and other securities, net in our consolidated statements of operations and comprehensive income. Our investments in interest-only and principal-only securities are included in agency MBS securities, at fair value on the accompanying consolidated balance sheets. We estimate the fair value of our investment securities based on a market approach using Level 2 inputs from third-party pricing services and dealer quotes. The third-party pricing services use pricing models that incorporate such factors as coupons, primary and secondary mortgage rates, prepayment speeds, spread to the Treasury and interest rate swap curves, convexity, duration, periodic and life caps and credit enhancements. The dealer quotes incorporate common market pricing methods, including a spread measurement to the Treasury or interest rate swap curve as well as underlying characteristics of the particular security including coupon, periodic and life caps, rate reset period, issuer, additional credit support and expected life of the security. We evaluate securities for other-than-temporary impairment (“OTTI”) on at least a quarterly basis, and more frequently when economic or market conditions warrant such evaluation. The determination of whether a security is other-than-temporarily impaired involves judgments and assumptions based on subjective and objective factors. When an investment security is impaired, an OTTI is considered to have occurred if (i) we intend to sell the investment security (i.e. a decision has been made as the reporting date) or (ii) it is more likely than not that we will be required to sell the investment security before recovery of its amortized cost basis. If we intend to sell the security or if it is more likely than not that we will be required to sell the investment security before recovery of its amortized cost basis, the entire amount of the impairment loss, if any, is recognized in earnings as a realized loss and the cost basis of the security is adjusted to its fair value. We did not recognize any OTTI charges on any of our investment securities for fiscal years 2011, 2010 and 2009 .
Interest Income	Interest Income Interest income is accrued based on the outstanding principal amount of the investment securities and their contractual terms. Premiums and discounts associated with the purchase of investment securities are amortized or accreted into interest income over the projected lives of the securities, including contractual payments and estimated prepayments using the interest method in accordance with ASC Subtopic 310-20, Receivables—Nonrefundable Fees and Other Costs (“ASC 310-20”). We estimate long-term prepayment speeds of our agency securities using a third-party service and market data. The third-party service estimates prepayment speeds using models that incorporate the forward yield curve, current mortgage rates, current mortgage rates of the outstanding loans, loan age, volatility and other factors. We review the prepayment speeds estimated by the third-party service and compare the results to market consensus prepayment speeds, if available. We also consider historical prepayment speeds and current market conditions to validate the reasonableness of the prepayment speeds estimated by the third-party service and based on our Manager’s judgment we may make adjustments to their estimates. Actual and anticipated prepayment experience is reviewed quarterly and effective yields are recalculated when differences arise between the previously estimated future prepayments and the amounts actually received plus current anticipated future prepayments. If the actual and anticipated future prepayment experience differs from our prior estimate of prepayments, we are required to record an adjustment in the current period to the amortization or accretion of premiums and discounts for the cumulative difference in the effective yield through the reporting date. The yield on our adjustable rate securities further assumes that the securities reset at a rate equal to the underlying index rate in effect as of the date we acquired the security plus the stated margin.
Repurchase Agreements	Repurchase Agreements We finance the acquisition of securities for our investment portfolio through repurchase transactions under master repurchase agreements. Pursuant to ASC Topic 860, Transfers and Servicing ("ASC 860"), we account for repurchase transactions as collateralized financing transactions, which are carried at their contractual amounts, including accrued interest, as specified in the respective transactions. Repurchase agreements are carried at cost, which approximates fair value due to their short-term nature.
Manager Compensation	Manager Compensation Our management agreement provides for the payment to our Manager of a management fee and reimbursement of certain operating expenses, which are accrued and expensed during the period for which they are earned or incurred. Refer to Note 7 for the terms of our management agreement and the administrative services agreement between American Capital and our Manager.
Derivative and other Hedging Instruments	Derivative and other Hedging Instruments We use a variety of derivative instruments to economically hedge a portion of our exposure to market risks, including interest rate and prepayment risk. The objective of our risk management strategy is to reduce fluctuations in net book value over a range of interest rate scenarios. In particular, we attempt to mitigate the risk of the cost of our variable rate liabilities increasing during a period of rising interest rates. The principal instruments that we use are interest rate swaps and options to enter into interest rate swaps (“interest rate swaptions”). We also purchase or sell to-be-announced agency MBS forward contracts (“TBAs”), specified agency MBS on a forward basis, U.S. Treasury securities and U.S. Treasury futures contracts. We may purchase or write put or call options on TBA securities and invest in other types of mortgage derivatives, such as interest-only securities, and synthetic total return swaps, such as the Markit IOS Synthetic Total Return Swap Index (“Markit IOS Index”). We account for derivative instruments in accordance with ASC Topic 815, Derivatives and Hedging (“ASC 815”). ASC 815 requires an entity to recognize all derivatives as either assets or liabilities in the balance sheet and to measure those instruments at fair value. Hedging instruments that are not derivatives under ASC 815 are accounted for as securities in accordance with ASC 320. The accounting for changes in the fair value of derivative instruments depends on whether the instruments are designated and qualify as part of a hedging relationship pursuant to ASC 815. Changes in fair value related to derivatives not in hedge designated relationships are recorded in gain (loss) on derivative instruments and other securities, net; whereas changes in fair value related to derivatives in hedge designated relationships are initially recorded in OCI and later reclassified to income at the time that the hedged transactions affect earnings. Any portion of the changes in fair value due to hedge ineffectiveness is immediately recognized in gain (loss) on derivative instruments and other securities, net. Derivatives in a gain position are reported as derivative assets at fair value and derivatives in a loss position are reported as derivative liabilities at fair value in our consolidated balance sheets. In our consolidated statements of cash flows, cash receipts and payments related to derivative instruments are classified according to the underlying nature or purpose of the derivative transaction, generally in the operating section for derivatives designated in hedging relationships and the investing section for derivatives not designated in hedging relationships. The use of derivatives creates exposure to credit risk relating to potential losses that could be recognized in the event that the counterparties to these instruments fail to perform their obligations under the contracts. We attempt to minimize this risk by limiting our counterparties to major financial institutions with acceptable credit ratings, monitoring positions with individual counterparties and adjusting posted collateral as required. Discontinuation of hedge accounting for interest rate swap agreements Prior to the third quarter of 2011, we entered into interest rate swap agreements typically with the intention of qualifying for hedge accounting under ASC 815. However, as of September 30, 2011, we elected to discontinue hedge accounting for our interest rate swaps in order to increase our funding flexibility. For our interest rate swaps to qualify as cash flow hedges pursuant to ASC 815, their monthly reset dates were required to align with the term of an underlying repurchase agreement. This alignment had the effect of limiting our ability to alter or extend the maturity of our repurchase agreements. To provide greater funding flexibility, we determined that it was not beneficial to always match the pricing dates of our swaps and repurchase agreements and, thus, elected to discontinue hedge accounting. Our net asset value was not impacted by our election to discontinue hedge accounting since our net asset value is the same irrespective of whether we apply hedge accounting. Upon discontinuation of hedge accounting, the net deferred loss related to our de-designated interest rate swaps remained in accumulated OCI. Following hedge de-designation, on September 30, 2011, the net deferred loss is reclassified from accumulated OCI into interest expense on a straight-line basis over the remaining term of each interest rate swap. However, although the reclassification of accumulated OCI into interest expense is similar to as if the interest rate swaps had not been de-designated, the actual net periodic interest costs associated with our de-designated interest rates swaps may be more or less than amounts reclassified into interest expense. The difference, as well as net periodic interest costs on interest rate swaps that were never in a hedge designation, along with subsequent changes in the fair value of our interest rates swaps, is reported in our consolidated statement of operations and comprehensive income in gain (loss) on derivative instruments and other securities, net. Cash flows from interest rate swaps subsequent to our discontinuance of hedge accounting are classified in investing activities on our consolidated statements of cash flows. For fiscal year 2011, subsequent to discontinuance of hedge accounting, we reclassified $53.6 million of net deferred losses from accumulated OCI into interest expense related to our de-designated interest rate swaps; whereas our total net periodic interest costs related to our de-designated interest rate swaps subsequent to discontinuance of hedge accounting was $82.8 million. The difference of $29.2 million and periodic interest rate costs related to interest rate swaps that we never placed in a hedge designation of $5.8 million (or $35.0 million combined) and other swap losses of $81.1 million are reported in our accompanying consolidated statement of operations and comprehensive income in gain (loss) on derivative instruments and other securities, net. In addition, for fiscal year 2011, we reported $53.6 million of other comprehensive income related to the reversal of deferred losses on de-designated interest rate swaps from accumulated OCI in our accompanying consolidated statement of operations and comprehensive income for the period subsequent to discontinuance of hedge accounting. As of December 31, 2011, the net deferred loss in accumulated OCI related to de-designated interest rate swaps was $690.8 million and the weighted average remaining contractual term was 3.2 years. The net deferred loss expected to be reclassified from OCI into interest expense over the next twelve months is $204.6 million. Interest rate swap agreements We use interest rate swaps to economically hedge the variable cash flows associated with short-term borrowings made under our repurchase agreement facilities. Under our interest rate swap agreements, we typically pay a fixed-rate and receive a floating rate based on one or three-month LIBOR with terms up to 10 years, which has the effect of modifying the repricing characteristics of our repurchase agreements and cash flows on such liabilities. We estimate the fair value of interest rate swaps based on inputs from a third-party pricing model. The third-party pricing model incorporates such factors as the Treasury curve, LIBOR rates, and the pay rate on the interest rate swaps. We also incorporate both our own and our counterparties’ nonperformance risk in estimating the fair value of our interest rate swap and swaption agreements. In considering the effect of nonperformance risk, we consider the impact of netting and credit enhancements, such as collateral postings and guarantees, and have concluded that our own and our counterparty risk is not significant to the overall valuation of these agreements. Interest rate swaptions We purchase interest rate swaptions to help mitigate the potential impact of increases or decreases in interest rates on the performance of our investment portfolio (referred to as “convexity risk”). The interest rate swaptions provide us the option to enter into an interest rate swap agreement for a predetermined notional amount, stated term and pay and receive interest rates in the future. The premium paid for interest rate swaptions is reported as an asset in our consolidated balance sheets. The premium is valued at an amount equal to the fair value of the swaption that would have the effect of closing the position adjusted for nonperformance risk, if any. The difference between the premium and the fair value of the swaption is reported in gain (loss) on derivative instruments and other securities, net in our consolidated statement of operations and comprehensive income. If a swaption expires unexercised, the loss on the swaption would be equal to the premium paid. If we sell or exercise a swaption, the realized gain or loss on the swaption would be equal to the difference between the cash or the fair value of the underlying interest rate swap received and the premium paid. We estimate the fair value of interest rate swaptions based on the fair value of the future interest rate swap that we have the option to enter into as well as the remaining length of time that we have to exercise the option. TBA securities A TBA security is a futures contract for the purchase or sale of agency MBS at a predetermined price, face amount, issuer, coupon and stated maturity on an agreed-upon future date. The specific agency MBS delivered into the contract upon the settlement date, published each month by the Securities Industry and Financial Markets Association, are not known at the time of the transaction. TBA securities are exempt from ASC 815 and are accounted for under ASC 320 if there is no other way to purchase or sell that security, if delivery of that security and settlement will occur within the shortest period possible for that type of security and if it is probable at inception and throughout the term of the individual contract that physical delivery of the security will occur (referred to as the “regular-way” exception). Alternatively, we may designate the TBA security as a qualifying cash flow hedge under ASC 815 if the regular-way exception is not met and at the time of the purchase or sale of the security, and throughout the term of the individual contract, it is probable that the forecasted transaction will occur and the hedging relationship is expected to be highly effective. For TBA security contracts that we have entered into, we have generally not asserted that physical settlement is probable or that the forecasted transaction is probable of occurring and, therefore, we typically have not designated these forward commitments as hedging instruments. Gains and losses associated with TBA contracts not subject to the regular-way exception or not designated as hedging instruments are recognized in our consolidated statement of operations and comprehensive income in gain (loss) on derivative instruments and other securities, net. We estimate the fair value of TBA securities based on similar methods used to our value agency MBS securities. Put and call options on TBA securities We may purchase put and call options on TBA securities to hedge against short-term changes in interest rates. Under a purchased put option, we have the right to sell to the counterparty a specified TBA security at a predetermined price on the option exercise date in exchange for a premium at execution. Under a purchased call option, we have the right to purchase from the counterparty a specified TBA security at a predetermined price on the option exercise date in exchange for a premium at execution. The premium paid for a put or call option is reported as an asset in our consolidated balance sheets. The premium is valued at an amount equal to the fair value of the option that would have the effect of closing the position adjusted for nonperformance risk, if any. The difference between the premium and the fair value of the option is reported in gain (loss) on derivative instruments and other securities, net in our consolidated statement of operations and comprehensive income. When a purchased put or call option expires unexercised, a realized loss is reported in our consolidated statement of operations and comprehensive income equal to the premium paid. When a purchased put or call option is exercised, a realized gain or loss is reported in our consolidated statement of operations and comprehensive income equal to the difference between the premium paid and the fair value of the exercised put or call option. In addition, a derivative asset is recorded in our consolidated balance sheet for the TBA security resulting from the put or call option exercise. We estimate the fair value of put and call options on TBA securities based on the fair value of the underlying TBA security as well as the remaining length of time to exercise the option. Forward commitments to purchase or sell specified agency MBS We enter into a forward commitment to purchase or sell specified agency MBS from time-to-time as a means of acquiring assets or as a hedge against short-term changes in interest rates. Contracts for the purchase or sale of specified agency MBS securities are accounted for as derivatives if the delivery of the specified agency MBS and settlement extends beyond the shortest period possible for that type of security. We may designate the forward commitment as a qualifying cash flow hedge if at the time of the purchase or sale of the security, and throughout the term of the individual contract, it is probable that physical delivery of the security will occur. Realized and unrealized gains and losses associated with forward commitments not designated as hedging instruments are recognized in our consolidated statement of operations and comprehensive income in gain (loss) on derivative instruments and other securities, net. We estimate the fair value of forward commitments to purchase or sell specified agency MBS based on similar methods used to value agency MBS as well as the remaining length of time of the forward commitment. U.S. Treasury securities We purchase or sell short U.S. Treasury securities and U.S. Treasury futures contracts to help mitigate the potential impact of changes in interest rates on the performance of our portfolio. We borrow securities to cover short sales of U.S. Treasury securities under reverse repurchase agreements. We account for these as securities borrowing transactions and recognize an obligation to return the borrowed securities at fair value on the balance sheet based on the value of the underlying borrowed securities as of the reporting date. Gains and losses associated with purchases and short sales of U.S. Treasury securities are recognized in gain (loss) on derivative instruments and other securities, net in our consolidated statements of operations and comprehensive income. Total return swaps We enter into total return swaps to obtain exposure to a security or market sector without owning such security or investing directly in that market sector. Total return swaps are agreements in which there is an exchange of cash flows whereby one party commits to make payments based on the total return (coupon plus the mark-to-market movement) of an underlying instrument or index in exchange for fixed or floating rate interest payments. To the extent the total return of the instrument or index underlying the transaction exceeds or falls short of the offsetting interest rate obligation, we will receive a payment from or make a payment to the counterparty. The primary total return swap index in which we invest is the Markit IOS Index. Total return swaps based on the Markit IOS index are intended to synthetically replicate the performance of interest-only securities. We determine the fair value of our total return swaps based on published index prices. Gains and losses associated with changes in market value of the underlying index and coupon interest are recognized in gain (loss) on derivative instruments and other securities, net in our consolidated statements of operations and comprehensive income.
Variable Interest Entities	Variable Interest Entities ASC Topic 810, Consolidation (“ASC 810”), requires a qualitative assessment in determining the primary beneficiary of a variable interest entity (“VIE”) and ongoing assessments of control over such entities as well as additional disclosures for entities that have variable interests in VIEs. We may enter into transactions involving a CMO trust (e.g. a VIE) whereby we transfer agency MBS to an investment bank in exchange for cash proceeds and at the same time enter into a commitment with the same investment bank to purchase to-be-issued securities collateralized by the agency MBS transferred. We will consolidate a CMO trust (as it relates to the assets transferred or contributed by us and the related liabilities issued by the trust) if we are the CMO trust’s primary beneficiary; that is, if we have a variable interest (or combination of variable interests) that provides us with a controlling financial interest in the CMO trust. An entity is deemed to have a controlling financial interest if the entity has the power to direct the activities of a VIE that most significantly impacts the VIE’s economic performance and the obligation to absorb losses of or right to receive benefits from the VIE that could potentially be significant to the VIE. In determining if we have a controlling financial interest, we evaluate whether we share the power to control the selection of financial assets transferred to the CMO trust with an unrelated party. We may share power in the selection of assets for certain CMO trusts (i.e., both we and the unrelated party must consent to the transfer of such assets to the CMO trust); however, if our economic interest in the CMO trust is disproportionate to the shared power, we may be deemed to be the primary beneficiary.
Income Tax, Policy [Policy Text Block]	Income Taxes We elected to be taxed as a REIT under the provisions of the Internal Revenue Code and the corresponding provisions of state law, commencing with our initial tax year ended December 31, 2008. In order to qualify as a REIT, we must annually distribute, in a timely manner to our stockholders, at least 90% of our taxable ordinary income. A REIT is not subject to tax on its earnings to the extent that it distributes its annual taxable income to its stockholders and as long as certain asset, income and stock ownership tests are met. We operate in a manner that will allow us to be taxed as a REIT. As permitted by the Internal Revenue Code, a REIT can designate dividends paid in the subsequent year as dividends of the current year if those dividends are both declared by the extended due date of the REIT's federal income tax return and paid to stockholders by the last day of the subsequent year. As a REIT, if we fail to distribute in any calendar year at least the sum of (i) 85% of our ordinary income for such year, (ii) 95% of our capital gain net income for such year and (iii) any undistributed taxable income from the prior year, we are subject to a non-deductible 4% excise tax on the excess of such required distribution over the sum of (a) the amounts actually distributed and, if applicable, (b) the amounts of income we retained and on which we have paid corporate income tax. Dividends declared by December 31 and paid by January 31 are treated as having been a distribution of our taxable income for the prior tax year. We and our domestic subsidiary, American Capital Agency TRS, LLC, have made a joint election to treat our subsidiary as a taxable REIT subsidiary. As such, American Capital Agency TRS, LLC, is subject to federal and state income tax. We evaluate uncertain income tax positions, if any, in accordance with ASC Topic 740, Income Taxes (“ASC 740”). To the extent we incur interest and/or penalties in connection with our tax obligations, such amounts shall be classified as income tax expense on our consolidated statements of operations.
Reclassifications	Reclassifications Certain prior period amounts in the consolidated financial statements have been reclassified to conform to the current period presentation.
Discontinuation of Election to Account for Interest Rate Swaps as Designated Cash Flow Hedges [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Derivative and other Hedging Instruments	Interest rate swap agreements We use interest rate swaps to economically hedge the variable cash flows associated with short-term borrowings made under our repurchase agreement facilities. Under our interest rate swap agreements, we typically pay a fixed-rate and receive a floating rate based on one or three-month LIBOR with terms up to 10 years, which has the effect of modifying the repricing characteristics of our repurchase agreements and cash flows on such liabilities. We estimate the fair value of interest rate swaps based on inputs from a third-party pricing model. The third-party pricing model incorporates such factors as the Treasury curve, LIBOR rates, and the pay rate on the interest rate swaps. We also incorporate both our own and our counterparties’ nonperformance risk in estimating the fair value of our interest rate swap and swaption agreements. In considering the effect of nonperformance risk, we consider the impact of netting and credit enhancements, such as collateral postings and guarantees, and have concluded that our own and our counterparty risk is not significant to the overall valuation of these agreements.