Intangible asset amortization expense amounted to $12 million, $4 million and $4 million for the years ended December 31, 2018, 2017 and 2016, respectively.
Estimated amortization expense for the next five years subsequent to December 31, 2018 is as follows:
## Table 0 ##
Note 9: Shareholders’ Equity Common Stock Under the dividend reinvestment and direct stock purchase plan (the “DRIP”), shareholders may reinvest cash dividends and purchase additional Company common stock, up to certain limits, through the plan administrator without commission fees.
Shares purchased by participants through the DRIP may be newly issued shares, treasury shares, or at the Company’s election, shares purchased by the plan administrator in the open market or in privately negotiated transactions.
Purchases generally will be made and credited to DRIP accounts once each week.
As of December 31, 2018, there were approximately 4.2 million shares available for future issuance under the DRIP.
Anti-dilutive Stock Repurchase Program In February 2015, the Company’s Board of Directors authorized an anti-dilutive stock repurchase program, which allowed the Company to purchase up to 10 million shares of its outstanding common stock over an unrestricted period of time.
The Company repurchased 0.6 million shares and 0.7 million shares of common stock in the open market at an aggregate cost of $45 million and $54 million under this program for the years ended December 31, 2018 and 2017, respectively.
As of December 31, 2018, there were 5.5 million shares of common stock available for purchase under the program.
The following table provides the number of water and wastewater customers we served by class as of December 31:
## Table 1 ##
(a) Includes public authorities and other utilities and community water and wastewater systems under bulk contracts.
Bulk contracts, which are accounted for as a single customer in the table above, generally result in service to multiple customers.
Customer growth in our Regulated Businesses is primarily from (i) adding new customers to our customer base through acquisitions of water and/or wastewater utility systems, (ii) organic population growth in our authorized service areas, and (iii) sale of water to other water utilities and community water systems.
Capital Investment The aging water and wastewater infrastructure in the United States is in need of replacement and modernization.
The American Society of Civil Engineers published its Report Card for America’s Infrastructure in 2017 and gave the drinking water industry a grade of “D,” and the wastewater infrastructure a grade of “D+,” primarily because much of the country’s infrastructure is nearing the end of its useful life.
The report also stated that, on an annual basis, approximately 2 trillion gallons of treated water are lost in the United States through more than 240,000 main breaks, at an estimated cost of $2.6 billion.
This represents approximately 20% of all treated water in the country.
In addition, approximately 900 billion gallons of untreated sewage is discharged into the country’s rivers and streams every year.
As documented in the American Water Works Association’s Buried No Longer: Confronting America’s Water Infrastructure Challenge, restoring the country’s existing water systems as they reach the end of their useful lives, and expanding them to serve a growing population, will cost at least $1 trillion between 2011 and 2035, just to maintain the current levels of water service.
In addition, increased regulation related to water quality and the management of water and wastewater residual discharges, which began with the passage of the Clean Water Act in 1972 and the Safe Drinking Water Act in 1974, is also a primary driver for the need to modernize the country’s water and wastewater infrastructure.
To address the growing infrastructure needs in the areas we serve, we plan to invest $7.3 billion over the next five years, including $1.6 billion in 2019, for capital improvements to our Regulated Businesses’ water and wastewater infrastructure, largely for pipe replacement and upgrading aging water and wastewater treatment facilities.
We have proactively improved our pipe renewal rate from a 250-year replacement cycle to a 120-year replacement cycle, which we anticipate will enable us to replace nearly 2,000 miles of mains and collection pipes between 2019 and 2023.
In addition, during this same five-year period, our capital investment in treatment plants, storage tanks and other key, above-ground facilities has increased over 10%, further addressing infrastructure renewal, resiliency, water quality, operational efficiency, technology and innovation, and emerging regulatory compliance needs.
Additionally, we are investing significantly in resiliency projects to address the impacts of climate and weather variability by hardening our assets.
Economic Regulation and Rate Making Our Regulated Businesses operate under a regulatory compact whereby, in exchange for exclusive rights to provide water and wastewater services in defined service territories, we have an obligation to serve customers within those territories requesting service, within reasonable limits.
In return for agreeing to invest capital into our water and wastewater systems, we are given the opportunity to recover our costs of doing business and earn a reasonable rate of return on our investments.
In 2018, net income attributable to common shareholders increased $141 million, or 33.1 percent, compared to the same period in 2017.
This increase was due to a $125 million non-cash charge in 2017 related to the implementation of TCJA, continued growth in the Regulated Businesses, driven by infrastructure investment, acquisitions and organic growth and growth in the Market-Based Businesses, mainly from our Homeowner Services Group due to the mid-year acquisition of Pivotal.
These increases were partially offset by higher O&M and depreciation expenses across the Company due to growth of the business and the asset impairment charge recorded for our Keystone subsidiary during the third quarter of 2018.
In 2017, net income attributable to common shareholders decreased $42 million, or 9.0 percent, as compared to the same period in 2016.
This decrease was due to a $125 million non-cash charge in 2017 related to the implementation of TCJA resulting from the re-measurement of deferred taxes primarily at the parent company from the reduction in the federal corporate income tax rate from 35% to 21%.
Partially offsetting this non-cash charge was continued growth in the Regulated Businesses driven mainly by infrastructure investment, acquisitions and organic growth, combined with growth in the Market-Based Businesses mainly from the Homeowner Services Group and Keystone.
Segment Results of Operations Our operating segments are comprised of the revenue-generating components of the business for which separate financial information is internally produced and regularly used by management to make operating decisions, assess performance and allocate resources.
The Company operates its business primarily through one reportable segment, the Regulated Businesses segment.
We also operate several market-based businesses within operating segments that individually do not meet the criteria of a reportable segment in accordance with GAAP.
These non-reportable operating segments are collectively presented as our Market-Based Businesses, which is consistent with how management assesses the results of these businesses.
Regulated Businesses Segment The following table provides financial information for our Regulated Businesses:
## Table 2 ##
invest between a range of $1.7 billion to $1.8 billion in 2019, with $1.6 billion for infrastructure improvements in our Regulated Businesses.
Also in 2019, we expect to invest between $120 million to $240 million for acquisitions in our Regulated Businesses.
Cash Flows from Financing Activities The following table provides a summary of the major items affecting our cash flows provided by financing activities:

<table><tr><td></td><td>Amount</td></tr><tr><td>2019</td><td>$15</td></tr><tr><td>2020</td><td>13</td></tr><tr><td>2021</td><td>11</td></tr><tr><td>2022</td><td>10</td></tr><tr><td>2023</td><td>7</td></tr></table>
<table><tr><td></td><td colspan="2">2018</td><td colspan="2">2017</td><td colspan="2">2016</td></tr><tr><td>(In thousands)</td><td>Water</td><td>Wastewater</td><td>Water</td><td>Wastewater</td><td>Water</td><td>Wastewater</td></tr><tr><td>Residential</td><td>2,892</td><td>188</td><td>2,872</td><td>182</td><td>2,846</td><td>171</td></tr><tr><td>Commercial</td><td>222</td><td>11</td><td>221</td><td>11</td><td>220</td><td>10</td></tr><tr><td>Fire service</td><td>48</td><td>—</td><td>47</td><td>—</td><td>45</td><td>—</td></tr><tr><td>Industrial</td><td>4</td><td>—</td><td>4</td><td>—</td><td>4</td><td>—</td></tr><tr><td>Public and other(a)</td><td>16</td><td>1</td><td>16</td><td>—</td><td>16</td><td>—</td></tr><tr><td>Total</td><td>3,182</td><td>200</td><td>3,160</td><td>193</td><td>3,131</td><td>181</td></tr></table>
<table><tr><td></td><td colspan="3">For the Years Ended December 31,</td><td colspan="2" rowspan="2"Increase (Decrease) 2018 vs. 2017></td><td colspan="2" rowspan="2"Increase (Decrease) 2017 vs. 2016></td></tr><tr><td></td><td>2018</td><td>2017</td><td>2016</td></tr><tr><td>(Dollars in millions)</td><td></td><td></td><td></td><td>$</td><td>%</td><td>$</td><td>%</td></tr><tr><td>Operating revenues</td><td>$2,984</td><td>$2,958</td><td>$2,871</td><td>$26</td><td>0.9</td><td>$87</td><td>3.0</td></tr><tr><td>Operation and maintenance</td><td>1,159</td><td>1,076</td><td>1,165</td><td>83</td><td>7.7</td><td>-89</td><td>-7.6</td></tr><tr><td>Depreciation and amortization</td><td>500</td><td>462</td><td>440</td><td>38</td><td>8.2</td><td>22</td><td>5.0</td></tr><tr><td>General taxes</td><td>261</td><td>244</td><td>242</td><td>17</td><td>7.0</td><td>2</td><td>0.8</td></tr><tr><td>(Gain) on asset dispositions and purchases</td><td>-7</td><td>-16</td><td>-7</td><td>9</td><td>-56.3</td><td>-9</td><td>128.6</td></tr><tr><td>Interest, net</td><td>-280</td><td>-268</td><td>-256</td><td>-12</td><td>4.5</td><td>-12</td><td>4.7</td></tr><tr><td>Other income (expenses)</td><td>-247</td><td>-266</td><td>-257</td><td>19</td><td>-7.1</td><td>-9</td><td>3.5</td></tr><tr><td>Income before income taxes</td><td>826</td><td>925</td><td>775</td><td>-99</td><td>-10.7</td><td>150</td><td>19.4</td></tr><tr><td>Provision for income taxes</td><td>224</td><td>367</td><td>303</td><td>-143</td><td>-39.0</td><td>64</td><td>21.1</td></tr><tr><td>Net income attributable to common shareholders</td><td>602</td><td>559</td><td>472</td><td>43</td><td>7.7</td><td>87</td><td>18.4</td></tr></table>

Question: In the year with largest amount of Operating revenues, what's the increasing rate of Operation and maintenance?