AMERICAN TOWER CORPORATION AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS financial statements include impairment of long-lived assets (including goodwill), asset retirement obligations, revenue recognition, rent expense, stock-based compensation, income taxes and accounting for business combinations.
The Company considers events or transactions that occur after the balance sheet date but before the financial statements are issued as additional evidence for certain estimates or to identify matters that require additional disclosure.
Concentrations of Credit Risk—The Company is subject to concentrations of credit risk related to its cash and cash equivalents, notes receivable, accounts receivable, deferred rent asset and derivative financial instruments.
The Company mitigates its risk with respect to cash and cash equivalents and derivative financial instruments by maintaining its deposits and contracts at high quality financial institutions and monitoring the credit ratings of those institutions.
The Company derives the largest portion of its revenues, corresponding accounts receivable and the related deferred rent asset from a relatively small number of tenants in the telecommunications industry, and approximately 56% of its current year revenues are derived from four tenants.
In addition, the Company has concentrations of credit risk in certain geographic areas.
The Company mitigates its concentrations of credit risk with respect to notes and trade receivables and the related deferred rent assets by actively monitoring the credit worthiness of its borrowers and tenants.
In recognizing customer revenue, the Company must assess the collectibility of both the amounts billed and the portion recognized in advance of billing on a straight-line basis.
This assessment takes tenant credit risk and business and industry conditions into consideration to ultimately determine the collectibility of the amounts billed.
To the extent the amounts, based on management’s estimates, may not be collectible, recognition is deferred until such point as collectibility is determined to be reasonably assured.
Any amounts that were previously recognized as revenue and subsequently determined to be uncollectible are charged to bad debt expense included in Selling, general, administrative and development expense in the accompanying consolidated statements of operations.
Accounts receivable is reported net of allowances for doubtful accounts related to estimated losses resulting from a tenant’s inability to make required payments and allowances for amounts invoiced whose collectibility is not reasonably assured.
These allowances are generally estimated based on payment patterns, days past due and collection history, and incorporate changes in economic conditions that may not be reflected in historical trends, such as tenants in bankruptcy, liquidation or reorganization.
Receivables are written-off against the allowances when they are determined to be uncollectible.
Such determination includes analysis and consideration of the particular conditions of the account.
Changes in the allowances were as follows for the years ended December 31, (in thousands):
## Table 0 ##
Functional Currency—The functional currency of each of the Company’s foreign operating subsidiaries is the respective local currency, except for Costa Rica, where the functional currency is the U. S.  Dollar.
All foreign currency assets and liabilities held by the subsidiaries are translated into U. S.  Dollars at the exchange rate in
Noble Energy, Inc.
Supplemental Oil and Gas Information (Unaudited) 128 Proved Gas Reserves (Unaudited) The following reserves schedule was developed by our qualified petroleum engineers and sets forth the changes in estimated quantities of proved natural gas reserves:
## Table 1 ##
(1) Other International includes the North Sea, Ecuador (at December 31, 2009 and 2008), and China.
See Note 3.
Acquisitions and Divestitures and Note 4.
Asset Impairments.
(2) The 2009 negative revisions in the US are primarily due to lower year-end prices (224 Bcf), reclassifications of PUDs to probable reserves as a result of the SEC’s new five year development rule (75 Bcf), and increased lease operating expense and various well performance issues (98 Bcf).
The 2010 US revisions are a combination of increases from higher natural gas prices, which were more than offset by gas shrinkage from additional NGLs recorded in Wattenberg and the reclassification of 85 Bcf of PUDs to probable reserves, primarily in Wattenberg, as a result of the SEC's five year development rule.
Equatorial Guinea’s positive revisions in 2009 and 2010 are primarily due to additional production allowances related to LNG sales.
Israel’s revisions in 2010 reflect a change in the likelihood that the Noa field will be developed.
The 2011 US revisions were primarily associated with reclassification of vertical PUDs in Wattenberg which are no longer expected to be developed in five years due to shifting activity level from vertical to horizontal development and revisions to onshore dry-gas assets due to reduced activity assumptions, performance, and price.
International revisions are associated with performance revisions in the North Sea.
(3) The 2009 increase in US proved reserves is primarily due to ongoing low-risk development programs onshore in Wattenberg, the Rocky Mountain area, and East Texas.
The 2010 increase in US proved reserves is due to continuing development of onshore assets, primarily in the DJ Basin, Piceance Basin, and East Texas.
The 2010 increase in Israel is due to the recording of initial reserves at the Tamar development.
The 2011 increase in the US is primarily due to active development programs in the DJ Basin and the Marcellus Shale.
The increase in Israel was primarily due to continuing appraisal at Tamar and includes reserves for Noa which we have decided to develop.
(4) The increase relates to our Marcellus Shale asset acquisition in 2011 and our DJ Basin asset acquisition in 2010.
See Note 3.
Acquisitions and Divestitures.
(5) We sold non-core, onshore US assets in the Mid-Continent and Illinois Basin in 2010.
Other International sales in 2010 include 160 Bcf due to the termination of the Block 3 PSC by the Ecuadorian government.
Table of Contents Index to Financial Statements 78 Available Liquidity Year-end liquidity was as follows:
## Table 2 ##
(1) Total cash includes cash and cash equivalents of almost $1.2 billion, which includes $57 million cash relating to Noble Midstream Partners, as well as restricted cash of $30 million related to the Permian Basin property acquisition that closed in January 2017.
(2) Excludes $350 million available to be borrowed under Noble Midstream Services Revolving Credit Facility, which is not available to Noble Energy, as of December 31, 2016.
See Revolving Credit Facilities, below.
(3) Total debt includes capital lease and other obligations and excludes unamortized debt discount/premium, and issuance costs.
(4) We define our ratio of debt-to-book capital as total debt (which includes long-term debt excluding unamortized discount/premium and issuance costs, the current portion of long-term debt, and short-term borrowings) divided by the sum of total debt plus Noble Energy's share of equity.
Cash and Cash Equivalents We had approximately $1.2 billion in cash and cash equivalents at December 31, 2016, compared with approximately $1.0 billion at December 31, 2015.
At December 31, 2016, our cash was primarily denominated in US dollars and invested in money market funds and short-term deposits with major financial institutions.
Approximately $435 million of this cash was attributable to foreign subsidiaries.
We have recorded a related deferred tax liability on undistributed foreign earnings of $241 million for the future additional US tax liability for the US and foreign tax rate differences, net of estimated foreign tax credits.
Our cash on hand at December 31, 2016 also included $57 million relating to Noble Midstream Partners.
Revolving Credit Facilities Noble Energy's Revolving Credit Facility of $4.0 billion committed amount matures in 2020.
We expect to use the Revolving Credit Facility to fund our capital investment program and the acquisition of Clayton Williams Energy, Inc. , and may periodically borrow amounts for working capital purposes.
Noble Midstream Services' Revolving Credit Facility of $350 million committed amount matures in 2021.
No amounts were drawn under either of these facilities at December 31, 2016.
See Item 8.
Financial Statements and Supplementary Data - Note 10.
Long-Term Debt.
Term Loan We occasionally access the capital markets to ensure adequate liquidity exists in the form of unutilized capacity under our Revolving Credit Facility or to refinance scheduled debt maturities.
In first quarter 2016, we entered into the Term Loan Facility which provides for a three-year term loan facility for a principal amount of $1.4 billion.
In connection with the Term Loan Facility, we launched cash tender offers for certain senior notes assumed in the Rosetta Merger.
The borrowings under the Term Loan Facility were used solely to fund the tender offers, which resulted in a gain of $80 million.
Collectively, the result of these transactions provides for significant future interest expense savings with a shorter term debt maturity.
In fourth quarter 2016, we prepaid $850 million of long-term debt outstanding under the Term Loan Facility from cash on hand.
See Item 8.
Financial Statements and Supplementary Data - Note 10.
Long-Term Debt.
Activities Enhancing Our Liquidity Position We were able to employ the above strategy successfully in 2016, taking steps to further position us for long-term operational performance and future growth even in a period of lower commodity prices.
The following activities have further enhanced our liquidity position: Asset Divestitures We evaluate potential strategic farm-out arrangements of our working interests for reimbursement of our capital spending and may consider asset sales or other sources of funding.
We generated cash proceeds of almost $1.2 billion in 2016, $151 million in 2015 and $321 million in 2014 from asset sales and farm-out arrangements.
Noble Midstream Partners IPO On September 20, 2016, our subsidiary, Noble Midstream Partners, completed its public offering of 14,375,000 common units representing limited partner interests in Noble Midstream Partners.
In connection with the offering, Noble Midstream Partners generated net proceeds of $299 million from the issuance of common units.
As of December 31, 2016, we owned 1,527,584 of Noble Midstream Partners common units and 15,902,584 of Noble Midstream Partners subordinated units, which together represented a 54.8% limited partner interest.

<table><tr><td></td><td>2014</td><td>2013</td><td>2012</td></tr><tr><td>Balance as of January 1</td><td>$19,895</td><td>$20,406</td><td>$24,412</td></tr><tr><td>Current year increases</td><td>8,243</td><td>7,025</td><td>8,028</td></tr><tr><td>Write-offs, net of recoveries and other</td><td>-10,832</td><td>-7,536</td><td>-12,034</td></tr><tr><td>Balance as of December 31</td><td>$17,306</td><td>$19,895</td><td>$20,406</td></tr></table>
<table><tr><td></td><td colspan="5">Natural Gas and Casinghead Gas (Bcf)</td></tr><tr><td></td><td>United States</td><td>Equatorial Guinea</td><td>Israel</td><td>Other Int'l -1</td><td>Total</td></tr><tr><td>Proved Reserves as of:</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>December 31, 2008</td><td>1,859</td><td>978</td><td>273</td><td>205</td><td>3,315</td></tr><tr><td>Revisions of Previous Estimates-2</td><td>-397</td><td>49</td><td>-2</td><td>-</td><td>-350</td></tr><tr><td>Extensions, Discoveries and Other Additions-3</td><td>211</td><td>-</td><td>5</td><td>2</td><td>218</td></tr><tr><td>Purchase of Minerals in Place-4</td><td>6</td><td>-</td><td>-</td><td>-</td><td>6</td></tr><tr><td>Sale of Minerals in Place-5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Production</td><td>-145</td><td>-87</td><td>-42</td><td>-11</td><td>-285</td></tr><tr><td>December 31, 2009</td><td>1,534</td><td>940</td><td>234</td><td>196</td><td>2,904</td></tr><tr><td>Revisions of Previous Estimates-2</td><td>-6</td><td>12</td><td>-41</td><td>-3</td><td>-38</td></tr><tr><td>Extensions, Discoveries and Other Additions-3</td><td>140</td><td>-</td><td>1,698</td><td>-</td><td>1,838</td></tr><tr><td>Purchase of Minerals in Place-4</td><td>139</td><td>-</td><td>-</td><td>-</td><td>139</td></tr><tr><td>Sale of Minerals in Place-5</td><td>-35</td><td>-</td><td>-</td><td>-160</td><td>-195</td></tr><tr><td>Production</td><td>-146</td><td>-83</td><td>-47</td><td>-11</td><td>-287</td></tr><tr><td>December 31, 2010</td><td>1,626</td><td>869</td><td>1,844</td><td>22</td><td>4,361</td></tr><tr><td>Revisions of Previous Estimates-2</td><td>-241</td><td>7</td><td>-</td><td>-8</td><td>-242</td></tr><tr><td>Extensions, Discoveries and Other Additions-3</td><td>326</td><td>-</td><td>488</td><td>-</td><td>814</td></tr><tr><td>Purchase of Minerals in Place-4</td><td>406</td><td>-</td><td>-</td><td>-</td><td>406</td></tr><tr><td>Sale of Minerals in Place-5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Production</td><td>-141</td><td>-90</td><td>-63</td><td>-2</td><td>-296</td></tr><tr><td>December 31, 2011</td><td>1,976</td><td>786</td><td>2,269</td><td>12</td><td>5,043</td></tr><tr><td>Proved Developed Reserves as of:</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>December 31, 2008</td><td>1,268</td><td>700</td><td>216</td><td>201</td><td>2,385</td></tr><tr><td>December 31, 2009</td><td>1,114</td><td>638</td><td>191</td><td>192</td><td>2,135</td></tr><tr><td>December 31, 2010</td><td>1,156</td><td>597</td><td>145</td><td>19</td><td>1,917</td></tr><tr><td>December 31, 2011</td><td>1,195</td><td>497</td><td>83</td><td>11</td><td>1,786</td></tr><tr><td>Proved Undeveloped Reserves as of:</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>December 31, 2008</td><td>591</td><td>278</td><td>57</td><td>4</td><td>930</td></tr><tr><td>December 31, 2009</td><td>420</td><td>302</td><td>43</td><td>4</td><td>769</td></tr><tr><td>December 31, 2010</td><td>470</td><td>272</td><td>1,699</td><td>3</td><td>2,444</td></tr><tr><td>December 31, 2011</td><td>781</td><td>289</td><td>2,186</td><td>1</td><td>3,257</td></tr></table>
<table><tr><td></td><td colspan="3">December 31,</td></tr><tr><td></td><td>2016</td><td>2015</td><td>2014</td></tr><tr><td>(millions, except percentages)</td><td></td><td></td><td></td></tr><tr><td>Total Cash<sup>-1</sup></td><td>$1,209</td><td>$1,028</td><td>$1,183</td></tr><tr><td>Amount Available to be Borrowed Under Revolving Credit Facility<sup>-2</sup></td><td>4,000</td><td>4,000</td><td>4,000</td></tr><tr><td>Total Liquidity</td><td>$5,209</td><td>$5,028</td><td>$5,183</td></tr><tr><td>Total Debt<sup>-3</sup></td><td>$7,114</td><td>$7,976</td><td>$6,197</td></tr><tr><td>Noble Energy Share of Equity</td><td>9,600</td><td>10,370</td><td>10,325</td></tr><tr><td>Ratio of Debt-to-Book Capital<sup>-4</sup></td><td>43%</td><td>43%</td><td>38%</td></tr></table>

Question: for the four largest tenants , what is the average % ( % ) of current year revenues that each represents?
Answer: 14