<?xml version="1.0" encoding="utf-8"?>
<InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.2.0.7</Version>
  <hasSegments>false</hasSegments>
  <ReportName>Fair Value Measurements (Policies)</ReportName>
  <ReportLongName>0403 - Disclosure - Fair Value Measurements (Policies)</ReportLongName>
  <DisplayLabelColumn>true</DisplayLabelColumn>
  <ShowElementNames>false</ShowElementNames>
  <RoundingOption />
  <HasEmbeddedReports>false</HasEmbeddedReports>
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="6 Months Ended" />
        <Label Id="2" Label="Jun. 30, 2010" />
      </Labels>
      <CurrencyCode>USD</CurrencyCode>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <UnitID>USD</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema>
            <MeasureValue>USD</MeasureValue>
            <MeasureNamespace>iso4217</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>Shares</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>shares</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>Pure</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>pure</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <UnitID>USDEPS</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>
        </Unit>
      </Units>
      <CurrencySymbol>$</CurrencySymbol>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>Fair Value Measurements (Policies) [Abstract]</Label>
      <Level>0</Level>
      <ElementName>so_FairValueMeasurementsPoliciesAbstract</ElementName>
      <ElementPrefix>so</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>Fair Value Measurements.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>xbrli:stringItemType</ElementDataType>
      <SimpleDataType>string</SimpleDataType>
      <ElementDefenition>Fair Value Measurements.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>Valuation Methodologies</Label>
      <Level>1</Level>
      <ElementName>so_ValuationMethodologiesPolicyTextBlock</ElementName>
      <ElementPrefix>so</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>Valuation Methodologies.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole>verboselabel</PreferredLabelRole>
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText>&lt;!--DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" --&gt;
   &lt;!-- Begin Block Tagged Accounting Policy: SO-20100630_note3_accounting_policy_table1 - so:ValuationMethodologiesPolicyTextBlock--&gt;
   &lt;div style="font-family: 'Times New Roman',Times,serif"&gt;
   &lt;div style="margin-top: 6pt"&gt;
   &lt;table width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt; text-align: left"&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="2%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="2%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&lt;i&gt;&amp;#160;&lt;/i&gt;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&lt;b&gt;&lt;i&gt;&amp;#160;&lt;/i&gt;&lt;/b&gt;&lt;/td&gt;
       &lt;td&gt;&lt;b&gt;&lt;i&gt;Valuation Methodologies&lt;/i&gt;&lt;/b&gt;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 6pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="2%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="2%" nowrap="nowrap" align="left"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The energy-related derivatives primarily consist of over-the-counter financial products for
   natural gas and physical power products, including from time to time, basis swaps. These are
   standard products used within the energy industry and are valued using the market approach. The
   inputs used are mainly from observable market sources, such as forward natural gas prices, power
   prices, implied volatility, and LIBOR interest rates. Interest rate derivatives are also
   standard over-the-counter financial products valued using the market approach. Inputs include
   LIBOR interest rates, interest rate futures contracts, and occasionally implied volatility of
   interest rate options. See Note (H)&amp;#160;herein for additional information on how these derivatives
   are used.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 6pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="2%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="2%" nowrap="nowrap" align="left"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;&amp;#8220;Other investments&amp;#8221; include investments in funds that are valued using the market approach and
   income approach. Securities that are traded in the open market are valued at the closing price
   on their principal exchange as of the measurement date. Discounts are applied in accordance
   with GAAP when certain trading restrictions exist. For investments that are not traded in the
   open market, the price paid will have been determined based on market factors including
   comparable multiples and the expectations regarding cash flows and business plan execution. As
   the investments mature or if market conditions change materially, further analysis of the fair
   market value of the investment is performed. This analysis is typically based on a metric, such
   as multiple of earnings, revenues, earnings
   before interest and income taxes, or earnings adjusted for certain cash changes. These
   multiples are based on comparable multiples for publicly traded companies or other relevant
   prior transactions.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 6pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="2%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="2%" nowrap="nowrap" align="left"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;For fair value measurements of investments within the nuclear decommissioning trusts and rabbi
   trust funds, specifically the fixed income assets using significant other observable inputs and
   significant unobservable inputs, the primary valuation technique used is the market approach.
   External pricing vendors are designated for each of the asset classes in the nuclear
   decommissioning trusts and rabbi trust funds with each security discriminately assigned a
   primary pricing source, based on similar characteristics.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 6pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="2%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="2%" nowrap="nowrap" align="left"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;A market price secured from the primary source vendor is then used in the valuation of the
   assets within the trusts. As a general approach, market pricing vendors gather market data
   (including indices and market research reports) and integrate relative credit information,
   observed market movements, and sector news into proprietary pricing models, pricing systems, and
   mathematical tools. Dealer quotes and other market information including live trading levels
   and pricing analysts&amp;#8217; judgment are also obtained when available.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;/table&gt;
   &lt;/div&gt;
   &lt;/div&gt;
</NonNumbericText>
          <NonNumericTextHeader>&lt;!--DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" --&gt;
   &lt;!-- Begin Block Tagged</NonNumericTextHeader>
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>us-types:textBlockItemType</ElementDataType>
      <SimpleDataType>textblock</SimpleDataType>
      <ElementDefenition>Valuation Methodologies.</ElementDefenition>
      <ElementReferences>No authoritative reference available.</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <NumberOfCols>1</NumberOfCols>
  <NumberOfRows>2</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
