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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-03-03</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001441849</issuerCik>
        <issuerName>CLOUD PEAK ENERGY INC.</issuerName>
        <issuerTradingSymbol>CLD</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001477110</rptOwnerCik>
            <rptOwnerName>MARSHALL COLIN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>505 S. GILLETTE AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>GILLETTE</rptOwnerCity>
            <rptOwnerState>WY</rptOwnerState>
            <rptOwnerZipCode>82716</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President &amp; CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-03-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>200437</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>200437</value>
                    <footnoteId id="F2"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200437</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each restricted stock unit (&quot;RSU&quot;) represents a contingent right to receive the value of one share of Cloud Peak Energy Inc. (&quot;CPE&quot;) common stock.  In general, 100% of the RSUs vest three years after the date of grant subject to the terms of the award agreement and CPE's long-term incentive plan (&quot;LTIP&quot;).  Vested RSUs may be paid in shares, cash or a combination of shares and cash, in the discretion of the Compensation Committee.</footnote>
        <footnote id="F2">This total does not include performance share units (&quot;PSUs&quot;) that were also granted on March 3, 2017 and that will be reported under Section 16 upon any vesting, as provided by applicable rules.  PSUs represent a contingent right to receive the value of one share of CPE common stock.  In general, PSUs vest 3 years from the date of grant depending on CPE's achievement of performance goals, which are based upon CPE's relative and absolute total shareholder return (&quot;TSR&quot;), and subject to the terms of the award agreement and CPE's LTIP.  These TSR achievements may result in a payout ranging between 0% and 200% of the target.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Bryan J. Pechersky, attorney-in-fact for Colin Marshall</signatureName>
        <signatureDate>2017-03-06</signatureDate>
    </ownerSignature>
</ownershipDocument>
