<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2021-11-01</periodOfReport>

    <issuer>
        <issuerCik>0000318154</issuerCik>
        <issuerName>AMGEN INC</issuerName>
        <issuerTradingSymbol>AMGN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001798821</rptOwnerCik>
            <rptOwnerName>Griffith Peter H.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE AMGEN CENTER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>THOUSAND OAKS</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91320</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>EVP &amp; CFO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2021-11-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>2661</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>206.97</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23700</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">These shares include the following RSUs granted under the Company's equity plans: 12,296 RSUs which vest in one installment of 6,056 on 11/1/2022 and one installment of 6,240 on 11/1/2023; 3,384 RSUs which vest in installments of 1,116 on 5/5/2022, 1,117 on 5/5/2023 and 1,151 on 5/5/2024; and 3,338 RSUs which vest in installments of 1,101 on 4/30/2023, 1,102 on 4/30/2024 and 1,135 on 4/30/2025. Vested RSUs will be paid in shares of the Company's common stock on a one-to-one basis.</footnote>
        <footnote id="F2">These shares include 930 Dividend Equivalents (DEs) granted pursuant to the Amgen Inc. Amended and Restated 2009 Equity Incentive Plan and subject to a qualifying dividend reinvestment plan. DEs are credited on the reporting person's unvested RSUs and are paid out in shares of the Company's common stock on a one-to-one basis according to the vesting schedule, along with a cash payment for any remaining fractional amount.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Peter H. Griffith</signatureName>
        <signatureDate>2021-11-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
