We are proud of our successful focus on base business activity over the last five years, which now accounts for more than 90% of our revenues.
After an 18-month competitive process, LUMA Energy, our joint venture with ATCO, was selected for a 15-year operation and maintenance agreement to operate, maintain and modernize PREPA's more than 18,000 mile electric transmission and distribution system in Puerto Rico.
We grew our communication services revenue by more than 40%, meaningfully improved profit margins and ended the year with a total backlog of approximately $900 million.
We invested approximately $400 million in strategic acquisition of seven high-quality companies with great management teams that expand or enhance our ability to provide solutions to our customers, are additive to our base business and advance our strategic initiatives.
These companies add to our self-perform capabilities, which typically accounts for approximately 85% of our work and are key to providing cost certainty to our customers.
We strengthened our financial position with the closing of our $1 billion investment-grade senior notes offering, which we believe points to the resiliency and sustainability of our business model and positive multiyear outlook.
We demonstrated our commitment to stockholder value and confidence in Quanta's financial strength and continued growth opportunities through the acquisition of approximately $250 million of common stock and a 25% increase in our dividend.
Our utility customers, who account for more than 70% of our 2020 revenues, are leaders in the effort to reduce carbon emissions with aggressive efforts to modernize and harden their systems, expand their renewable generation portfolios and implement new technologies for current and future needs.
A number of utilities have committed to providing 100% of their power by clean energy or achieving net-zero carbon emissions by 2050.
According to a report from the WIRES Group, increased electrification and electric vehicle adoption in the United States could require 70 to 200 gigawatts of new power generation by 2030.
The majority of which is expected to be renewables and could require incremental transmission investment of $30 billion to $90 billion by 2030.
We expect to generate approximately $700 million in revenue this year, which would represent approximately 30% growth over 2020.
This fund provides more than $20 billion to bridge the digital divide that exist for millions of people living in rural America without access to adequate broadband connectivity.
Quanta is actively engaged in collaborative conversations with many of our customers about their multiyear, multibillion-dollar programs extending as far as 10 years, regarding how Quanta can provide solutions throughout our customers' value chain to meet their strategic infrastructure investment goals and support of these initiatives.
Today, we announced fourth quarter 2020 revenues of $2.9 billion.
Net income attributable to common stock was $170.1 million or $1.17 per diluted share and adjusted diluted earnings per share, a non-GAAP measure, was $1.22.
Our Electric Power revenues, excluding Latin America, were $2.11 billion, a 15.7% increase when compared to the fourth quarter of 2019.
This increase was driven by mid single-digit growth in our base business, increased contributions from the timing of certain larger projects and $75 million in revenues from acquired businesses.
Contributing to the base business growth was approximately 13% growth from our communications operations and record fourth quarter demand for our emergency restoration services of approximately $150 million, primarily associated with efforts to restore infrastructure in the Southeastern and Midwestern United States, although it came at the expense of certain other work in progress.
Electric segment margins in 4Q '20 were 11.6%.
And excluding our Latin American operations, segment margins were 12.9% versus 9% in 4Q '19.
Of note, our communications margins continue to improve against the prior year with a margin of 9% during the quarter.
As a reminder, we currently receive no tax benefit for losses in Latin America, so the $27 million in losses impacted the quarter by approximately $0.19.
Revenues from our underground utility and infrastructure Solutions segment were $806 million, 36% lower than 4Q '19.
Operating margins for the segment were 5.1%.
these margins were 190 basis points lower than 4Q '19, primarily due to reduced revenues as well as some degree of execution challenges during the quarter and costs associated with the exit of certain ancillary pipeline operations.
Our total backlog was $15.1 billion at the end of the fourth quarter, slightly higher than 4Q '19 and comparable to the third quarter of 2020, yet remains at record levels.
12-month backlog of $8.3 billion is an increase from both the fourth quarter of 2019 and the third quarter of 2020.
However, assuming an operating margin profile consistent with our Electric Power operations, LUMA's contribution over the 15-year operation and maintenance agreement would imply a backlog equivalent of more than $6 billion for Quanta.
For the fourth quarter of 2020, we generated free cash flow, a non-GAAP measure of $200 million.
And although $381 million lower than 4Q '19, it was higher than we anticipated, driven by stronger profits in the quarter and a cash cycle consistent with our third quarter results.
For the year, we generated record free cash flow of $892 million.
Days sales outstanding, or DSO, measured 83 days for the quarter, which was comparable to the third quarter of 2020 and fourth quarter of 2019.
Cash flows in the fourth quarter and full year 2020 did partially benefit from the deferral of $37 million and $109 million of employer payroll tax payments permitted by the CARES Act with the payments due in equal installment at the end of 2021 and 2022.
We had $185 million of cash at the end of the year with total liquidity of $2.2 billion and a debt-to-EBITDA ratio, as calculated under our credit agreement, of approximately 1.2 times.
Electric segment revenues grew to $7.8 billion at the end of 2020, and we continue to see our base business providing mid-single to double-digit growth opportunities, coupled with some degree of increased contributions from larger projects, primarily associated with previously announced projects in Canada.
In the aggregate, we expect Electric Power revenues to range between $8.3 billion and $8.5 billion, which includes expected revenues from our communications operations of around $700 million.
As it relates to Electric Power segment revenue seasonality, we expect revenue growth in each quarter of '21 compared to 2020, with quarter-over-quarter growth in the first and second quarters, potentially exceeding 10%.
We expect 2021 operating margins for the Electric Power segment to range between 10.1% and 10.9%, which includes contributions of approximately $29 million or $0.20 per share from the LUMA joint venture and earnings from other integral unconsolidated affiliates.
LUMA is expected to contribute around $9 million in the first half of the year and then increasing in the back half of the year as we exit the front-end transition services period.
Although we are proud of our overall Electric Power performance in 2020, our 11.6% margins, excluding Latin America, are above historical averages and are the highest since 2013, due in part to record annual emergency restoration service revenues of $450 million.
As outlined in our accompanying slides, our 2021 expectation for margins for this segment are consistent with historical averages and are also based on expectations for more normalized emergency restoration service revenues of approximately $200 million, also in line with historical averages.
As is typically the case, we expect that first quarter operating margins will be the lowest for the year, possibly slightly below 10%.
However, we are anticipating upper single-digit to double-digit revenue growth off of 2020 with full year revenues expected to range between $3.65 billion to $3.85 billion.
Over 90% of our revenue expectations for 2021 represent base business with larger projects representing their lowest level of contributions in the last seven years.
From a seasonality perspective, we see first quarter revenues being our lowest for the year, likely more than 20% lower than the first quarter of 2020.
We see segment margins ranging between 5.5% and 6%, led primarily by continued execution within our gas LDC operations.
However, to put our current segment margin guidance in context, if our industrial operations contributed at historical pre-COVID margin levels, our segment margin guidance would increase by over 100 basis points.
These segment operating ranges support our expectation for 2021 annual revenues of $11.95 billion to $12.35 billion, and adjusted EBITDA, a non-GAAP measure, of between $1.09 billion and $1.19 billion.
This represents 8% growth at the midpoint of the range when compared to 2020's record adjusted EBITDA.
With these operating results, we estimate our range of GAAP diluted earnings per share attributable to common stock for the year to be between $3.16 and $3.66, and anticipate non-GAAP adjusted diluted earnings per share to be between $4.02 and $4.52.
We expect free cash flow for 2021 to range between $400 million and $600 million with the standard disclaimer that quarterly free cash flow is subject to sizable movements due to various customer and project dynamics that occur in the normal course of operations.
Included in our free cash flow expectation is the anticipated payment of $54 million in the fourth quarter related to payroll taxes that were deferred in 2020.
For instance, a large driver of our significant free cash flow in 2020 was reduced revenues of approximately $900 million compared to 2019, decreasing working capital needs.
Looking back on our 2020 performance, although there were headwinds to the year, we ended the year with $11.2 billion in revenues, which represents an 8.1% revenue CAGR since 2015.
More importantly, we ended the year with slightly over $1 billion of adjusted EBITDA, a record for Quanta and equal to our goal established five years ago, which represents a nearly 15% CAGR since 2015.
Lastly, our record adjusted earnings per share of $3.82 represents a 28% CAGR since 2015, with our adjusted earnings per share growing faster than profits, which are growing faster than revenues.
Over the last five years, we have deployed approximately $1.4 billion in cash for M&A and strategic investments and $760 million for stock repurchases.
While we acquired $250 million of common stock in 2020 and $7 million of common stock through February 24, 2021, we have approximately $530 million of availability remaining on our current stock repurchase program.
Our $1 billion bond offering in 2020 established a fixed level of debt that nicely complements our current EBITDA profile, which we believe is a repeatable, sustainable baseline of earnings.

Summary:
Net income attributable to common stock was $170.1 million or $1.17 per diluted share and adjusted diluted earnings per share, a non-GAAP measure, was $1.22.
These segment operating ranges support our expectation for 2021 annual revenues of $11.95 billion to $12.35 billion, and adjusted EBITDA, a non-GAAP measure, of between $1.09 billion and $1.19 billion.
With these operating results, we estimate our range of GAAP diluted earnings per share attributable to common stock for the year to be between $3.16 and $3.66, and anticipate non-GAAP adjusted diluted earnings per share to be between $4.02 and $4.52.