A number of states have authorized the use of regulatory mechanisms that permit rates to be adjusted outside of a general rate case for certain costs and investments, such as infrastructure surcharge mechanisms that permit recovery of capital investments to replace aging infrastructure.
The following table details additional annualized revenues authorized through infrastructure surcharge mechanisms that were effective from 2013 through 2015:
## Table 0 ##
(a) Quarterly filings made with PUC in 2015 for $8 million, $4 million and $2 million effective October 1, 2015, July 1, 2015 and April 1, 2015, respectively.
No infrastructure charges in 2014 as a result of general rate case effective January 1, 2014 utilized forecasted test year and therefore qualifying infrastructure improvements already reflected in rates.
For 2013, $7 million, $4 million, $3 million and $6 million effective October 1, 2013, July 1, 2013, April 1, 2013 and January 1, 2013, respectively.
(b) Semi-annual filings made with the PUC.
For 2015, $9 million effective January 1, 2015.
For 2014, $7 million and $10 million effective July 1, 2014 and January 1, 2014, respectively.
For 2013, $4 million effective July 1, 2013.
(c) For 2015, $2 million effective June 27, 2015.
For 2014, $9 million and $4 million effective December 31, 2014 and May 30, 2014, respectively.
For 2013, $5 million and $3 million effective June 21, 2013 and December 14, 2013, respectively.
(d) Effective December 18, 2013.
(e) For 2015, $1 million and $5 million effective February 1, 2015 and January 1, 2015, respectively.
For 2014, $2 million effective January 1, 2014.
(f) For 2015, $1 million effective December 1, 2015.
For 2014, $1 million effective January 1, 2014 and March 3, 2014.
(g) For 2015, $2 million effective June 29, 2015.
For 2014, $1 million effective April 15, 2014.
On December 15, 2015, our Pennsylvania subsidiary filed for additional annualized revenues from infrastructure charges in the amount of $11 million which were authorized and effective on January 1, 2016.
On December 18, 2015, our Illinois subsidiary filed for additional annualized revenues from infrastructure charges in the amount of $1 million, which were authorized and also effective on January 1, 2016.
On January 14, 2016, our Indiana subsidiary filed for additional annualized revenues from infrastructure charges in the amount of $3 million.
As of February 24, 2016, we are awaiting final general rate case orders in five states, requesting additional annualized revenue of $123 million.
We are also awaiting approval of $5 million in additional annualized revenues for infrastructure charges.
There is no assurance that all or any portion of these requests will be granted.
Technology and Operational Efficiency – Continuing Improvement in O&M Efficiency Ratio for our Regulated Businesses We continued to improve on our O&M efficiency ratio.
Our adjusted O&M efficiency ratio for the year ended December 31, 2015 was 35.9%, compared to 36.7% for the year ended December 31, 2014.
The improvement in the 2015 O&M efficiency ratio over this period was attributable to both an increase in revenue and decreases in O&M expenses.
We evaluate our operating performance using this measure because management believes it is a direct measure of the efficiency of our Regulated Businesses’ operations.
This information is intended to enhance an
ÿþWe conduct our business primarily through one reportable segment, our Regulated Businesses segment.
 We  also operate businesses that provide a broad range of related and complementary water and wastewater services  in non-regulated markets, which includes four operating segments that individually do not meet the criteria of a  reportable segment.
 These four non-reportable segments have been combined and are presented as Market-Based  Businesses, which is consistent with how management assesses the results of our businesses.
  Regulated Businesses Segment  The following table summarizes certain financial information for our Regulated Businesses segment:  @T@  Operating revenues.
 The following tables and discussions provide explanation of the variances related to the  three components of operating revenues  water revenues, wastewater revenues and other revenues:  @T@  For the Years Ended December 31,  @T@  (a) The correlation between water service revenues and billed water sales volumes shown above is impacted by the California drought.
  California is experiencing a severe drought and in April 2015, the Governor mandated water usage restrictions to reduce overall water  usage by 25% in the state compared to 2013 levels.
 Our California customers are largely meeting these conservation targets.
 Revenue in  California is decoupled from sales volume, aligning our water conservation goals with those of the state and our customers and therefore  these usage reductions do not impact earnings.
  In 2015, water revenues increased $61 million, or 2.
4%, primarily due to a:  "  $45 million increase from rate increases and infrastructure charges;  "  $24 million increase from higher demand, mainly due to milder weather in 2014;
## Table 1 ##
## Table 2 ##
## Table 3 ##
Interest Rate Cash Flow Hedges – We report changes in the fair value of cash flow hedges in accumulated other comprehensive loss until the hedged item affects earnings.
At both December 31, 2008 and 2007, we had reductions of $4 million recorded as an accumulated other comprehensive loss that is being amortized on a straight-line basis through September 30, 2014.
As of December 31, 2008 and 2007, we had no interest rate cash flow hedges outstanding.
Earnings Impact – Our use of derivative financial instruments had the following impact on pre-tax income for the years ended December 31:
## Table 4 ##
Fair Value of Debt Instruments – The fair value of our short- and long-term debt was estimated using quoted market prices, where available, or current borrowing rates.
At December 31, 2008, the fair value of total debt is approximately $247 million less than the carrying value.
At December 31, 2007, the fair value of total debt exceeded the carrying value by approximately $96 million.
At December 31, 2008 and 2007, approximately $320 million and $181 million, respectively, of fixed-rate debt securities contained call provisions that allowed us to retire the debt instruments prior to final maturity, with the payment of fixed call premiums, or in certain cases, at par.
Sale of Receivables – The Railroad transfers most of its accounts receivable to Union Pacific Receivables, Inc.  (UPRI), a bankruptcy-remote subsidiary, as part of a sale of receivables facility.
UPRI sells, without recourse on a 364-day revolving basis, an undivided interest in such accounts receivable to investors.
The total capacity to sell undivided interests to investors under the facility was $700 million and $600 million at December 31, 2008 and 2007, respectively.
The value of the outstanding undivided interest held by investors under the facility was $584 million and $600 million at December 31, 2008 and 2007, respectively.
UPRI reduced the outstanding undivided interest held by investors due to a decrease in available receivables at December 31, 2008.
The value of the outstanding undivided interest held by investors is not included in our Consolidated Financial Statements.
The value of the undivided interest held by investors was supported by $1,015 million and $1,071 million of accounts receivable held by UPRI at December 31, 2008 and 2007, respectively.
At December 31, 2008 and 2007, the value of the interest retained by UPRI was $431 million and $471 million, respectively.
This retained interest is included in accounts receivable in our Consolidated Financial Statements.
The interest sold to investors is sold at carrying value, which approximates fair value, and there is no gain or loss recognized from the transaction.
The value of the outstanding undivided interest held by investors could fluctuate based upon the availability of eligible receivables and is directly affected by changing business volumes and credit risks, including default and dilution.
If default or dilution percentages were to increase one percentage point, the amount of eligible receivables would decrease by $6 million.
Should our credit rating fall below investment grade, the value of the outstanding undivided interest held by investors would be reduced, and, in certain cases, the investors would have the right to discontinue the facility.
The Railroad services the sold receivables; however, the Railroad does not recognize any servicing asset or liability as the servicing fees adequately compensate us for these responsibilities.
The Railroad collected approximately $17.8 billion and $16.1 billion during the years ended December 31, 2008 and 2007, respectively.
UPRI used certain of these proceeds to purchase new receivables under the facility.

<table><tr><td></td><td colspan="3">For the Years Ended December 31,</td></tr><tr><td></td><td>2015</td><td>2014</td><td>2013</td></tr><tr><td></td><td colspan="3">(In millions)</td></tr><tr><td>Infrastructure charges by state</td><td></td><td></td><td></td></tr><tr><td>Pennsylvania(a)</td><td>$14</td><td>$—</td><td>$20</td></tr><tr><td>New Jersey(b)</td><td>9</td><td>17</td><td>4</td></tr><tr><td>Missouri(c)</td><td>2</td><td>13</td><td>8</td></tr><tr><td>Indiana(d)</td><td>—</td><td>—</td><td>4</td></tr><tr><td>Illinois(e)</td><td>6</td><td>2</td><td>—</td></tr><tr><td>New York(f)</td><td>1</td><td>2</td><td>—</td></tr><tr><td>Tennessee(g)</td><td>2</td><td>1</td><td>—</td></tr><tr><td>Total Infrastructure charges</td><td>$34</td><td>$35</td><td>$36</td></tr></table>
<table><tr><td></td><td colspan="3">For the Years Ended December 31,</td></tr><tr><td>(In millions)</td><td>2015</td><td>2014</td><td>2013</td></tr><tr><td>Operating revenues</td><td>$2,743</td><td>$2,674</td><td>$2,594</td></tr><tr><td>Operation and maintenance</td><td>1,095</td><td>1,112</td><td>1,105</td></tr><tr><td>Operating expenses, net</td><td>1,732</td><td>1,726</td><td>1,700</td></tr><tr><td>Income from continuing operations</td><td>473</td><td>434</td><td>400</td></tr></table>
<table><tr><td></td><td colspan="3">For the Years Ended December 31,</td><td colspan="2" rowspan="2"Increase (Decrease) 2015 vs. 2014></td><td colspan="2" rowspan="2"Increase (Decrease) 2014 vs. 2013></td></tr><tr><td></td><td>2015</td><td>2014</td><td>2013</td></tr><tr><td>(In millions)</td><td colspan="3">Operating Revenues</td><td></td><td>%</td><td></td><td>%</td></tr><tr><td>Water service:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Residential</td><td>$1,536</td><td>$1,515</td><td>$1,465</td><td>$21</td><td>1.4</td><td>$50</td><td>3.4</td></tr><tr><td>Commercial</td><td>559</td><td>550</td><td>521</td><td>9</td><td>1.6</td><td>29</td><td>5.6</td></tr><tr><td>Industrial</td><td>130</td><td>132</td><td>119</td><td>-2</td><td>-1.5</td><td>13</td><td>10.9</td></tr><tr><td>Public and other</td><td>331</td><td>333</td><td>312</td><td>-2</td><td>-0.6</td><td>21</td><td>6.7</td></tr><tr><td>Other water revenues</td><td>39</td><td>26</td><td>18</td><td>13</td><td>50.0</td><td>8</td><td>44.4</td></tr><tr><td>Billed water services</td><td>2,595</td><td>2,556</td><td>2,435</td><td>39</td><td>1.5</td><td>121</td><td>5.0</td></tr><tr><td>Unbilled water services</td><td>-3</td><td>-25</td><td>30</td><td>22</td><td>-88.0</td><td>-55</td><td>-183.3</td></tr><tr><td>Total water revenues</td><td>$2,592</td><td>$2,531</td><td>$2,465</td><td>$61</td><td>2.4</td><td>$66</td><td>2.7</td></tr></table>
<table><tr><td></td><td colspan="3">For the Years Ended December 31,</td><td colspan="2" rowspan="2"Increase (Decrease) 2015 vs. 2014></td><td colspan="2" rowspan="2"Increase (Decrease) 2014 vs. 2013></td></tr><tr><td></td><td>2015</td><td>2014</td><td>2013</td></tr><tr><td>(In millions)</td><td colspan="3">Billed Water Sales Volume</td><td></td><td>%</td><td></td><td>%</td></tr><tr><td>Water service:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Residential(a)</td><td>175,653</td><td>176,975</td><td>180,976</td><td>-1,322</td><td>-0.7</td><td>-4,001</td><td>-2.2</td></tr><tr><td>Commercial</td><td>81,772</td><td>81,564</td><td>80,392</td><td>208</td><td>0.3</td><td>1,172</td><td>1.5</td></tr><tr><td>Industrial</td><td>38,991</td><td>39,833</td><td>37,107</td><td>-842</td><td>-2.1</td><td>2,726</td><td>7.3</td></tr><tr><td>Public and other</td><td>51,324</td><td>52,710</td><td>51,009</td><td>-1,386</td><td>-2.6</td><td>1,701</td><td>3.3</td></tr><tr><td>Billed water services</td><td>347,740</td><td>351,082</td><td>349,484</td><td>-3,342</td><td>-1.0</td><td>1,598</td><td>0.5</td></tr></table>
<table><tr><td><i>Millions of Dollars</i></td><td><i>2008</i></td><td><i>2007</i></td><td><i>2006</i></td></tr><tr><td>(Increase)/decrease in interest expense from interest rate hedging</td><td>$1</td><td>$-8</td><td>$-8</td></tr><tr><td>(Increase)/decrease in fuel expense from fuel derivatives</td><td>1</td><td>-1</td><td>3</td></tr><tr><td>Increase/(decrease) in pre-tax income</td><td>$2</td><td>$-9</td><td>$-5</td></tr></table>

Question: what was the difference in billions of sold receivables from 2007 to 2008?