<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>10401 - Disclosure - Accounts Receivable:</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2011 - 3/31/2011
USD ($)

USD ($) / shares
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>Duration_1_1_2011_To_3_31_2011</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000893949</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2011-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2011-03-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS><UnitProperty><UnitID>Unit12</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>Unit1</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>Unit13</UnitID><UnitType>Divide</UnitType><NumeratorMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></NumeratorMeasure><DenominatorMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></DenominatorMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="3 Months Ended" /><Label Id="2" Label="Mar. 31, 2011" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_AccountsReceivableNetAbstract</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole /><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>No definition available.</ElementDefenition><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Accounts Receivable:</Label></Row><Row><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTextBlock</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terselabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;

&lt;table style="border-collapse: collapse;" border="0" cellspacing="0" cellpadding="0" width="100%"&gt;
&lt;tr&gt;&lt;td valign="top" width="4%" align="left"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;4.&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="top" align="left"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;Accounts Receivable: &lt;/b&gt;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;

&lt;p style="margin-top: 6px; text-indent: 32px; margin-bottom: 0px; margin-left: 4%;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;Accounts receivable consists of the following (in thousands): &lt;/font&gt;&lt;/p&gt;

&lt;p style="margin-top: 0px; margin-bottom: 0px; font-size: 12px;"&gt;&amp;nbsp;&lt;/p&gt;

&lt;table border="0" cellspacing="0" cellpadding="0" width="76%" align="center"&gt;
&lt;tr&gt;&lt;td width="66%"&gt; &lt;/td&gt;
&lt;td valign="bottom" width="10%"&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;
&lt;td valign="bottom" width="10%"&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;
&lt;td&gt; &lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="border-bottom: #000000 1px solid;" valign="bottom" colspan="2" align="center"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="1"&gt;&lt;b&gt;March&amp;nbsp;31,&amp;nbsp;2011&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td style="border-bottom: #000000 1px solid;" valign="bottom" colspan="2" align="center"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="1"&gt;&lt;b&gt;December&amp;nbsp;31,&amp;nbsp;2010&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td height="8"&gt; &lt;/td&gt;
&lt;td height="8" colspan="4"&gt; &lt;/td&gt;
&lt;td height="8" colspan="4"&gt; &lt;/td&gt;&lt;/tr&gt;
&lt;tr bgcolor="#cceeff"&gt;&lt;td valign="top"&gt;

&lt;p style="text-indent: -1em; margin-left: 1em;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;Gross accounts receivable&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;642,209&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;603,372&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr&gt;&lt;td valign="top"&gt;

&lt;p style="text-indent: -1em; margin-left: 1em;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;Allowance for contractual adjustments and uncollectibles&lt;/font&gt;&lt;/p&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;(454,039&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;)&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;(421,977&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;)&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="font-size: 1px;"&gt;&lt;td valign="bottom"&gt; &lt;/td&gt;
&lt;td valign="bottom"&gt;&amp;nbsp;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 1px solid;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 1px solid;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td&gt;&amp;nbsp;&lt;/td&gt;
&lt;td valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 1px solid;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 1px solid;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td&gt;&amp;nbsp;&lt;/td&gt;&lt;/tr&gt;
&lt;tr bgcolor="#cceeff"&gt;&lt;td valign="top"&gt; &lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;188,170&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font class="_mt" size="1"&gt;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;181,395&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&amp;nbsp;&amp;nbsp;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;
&lt;tr style="font-size: 1px;"&gt;&lt;td valign="bottom"&gt; &lt;/td&gt;
&lt;td valign="bottom"&gt;&amp;nbsp;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 3px double;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 3px double;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td&gt;&amp;nbsp;&lt;/td&gt;
&lt;td valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 3px double;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td style="border-top: #000000 3px double;" valign="bottom"&gt;&amp;nbsp;&lt;/td&gt;
&lt;td&gt;&amp;nbsp;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt; &lt;/div&gt;</NonNumbericText><NonNumericTextHeader>4.
Accounts Receivable:

Accounts receivable consists of the following (in thousands):

&amp;nbsp;










</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>Disclosure itemizing the various types of trade accounts and notes receivable, and for each the gross carrying value, allowance, and net carrying value as of the balance sheet date. Presentation is categorized by current, noncurrent and unclassified receivables. This disclosure may include (1) the basis at which such receivables are carried in the entity's statements of financial position (2) how the level of the valuation allowance for receivables is determined (3) when impairments, charge-offs or recoveries are recognized for such receivables (4) the treatment of origination fees and costs, including the amortization method for net deferred fees or costs (5) the treatment of any premiums or discounts or unearned income (6) the entity's income recognition policies for such receivables, including those that are impaired, past due or placed on nonaccrual status and (7) the treatment of foreclosures or repossessions (8) the nature and amount of any guarantees to repurchase receivables.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 02
 -Paragraph 3, 4
 -Article 5

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