Backlog at the end of the quarter was a record $15.8 billion, which we believe reflects the continued advancement of our long-term growth strategies.
The recognition that the country's infrastructure needs to be modernized to support economic growth improved safety and reliability and for a cleaner environment is evidenced by the Biden administration's recently proposed $2 trillion infrastructure plan.
Additionally, we continue to believe, we can achieve at least $1 billion in annual revenue with double-digit operating income margins in the medium term.
We believe this demonstrates the strength and sustainability of our business and long-term strategy, favorable end market trends, our ability to safely execute, and our strong competitive position in the marketplace.
Today we announced first quarter 2021 revenues of $2.7 billion.
Net income attributable to common stock was $90 million or $0.62 per diluted share and adjusted diluted earnings per share a non-GAAP measure was $0.83.
Our electric power revenues were $2.1 billion a record for the first quarter and a 17% increase when compared to the first quarter of 2020.
This increase was driven by continued growth in base business activities as well as contributions from larger transmission projects under way in Canada and revenues from acquired businesses of approximately $70 million.
Also revenues associated with emergency restoration services attributable to winter storm response efforts were approximately $80 million a first quarter record.
Electric segment margins in 1Q 2021 were 9.7% versus 7.3% in 1Q 2020.
Operating margins also benefited from approximately $5 million of income associated with our LUMA joint venture.
Underground Utility and Infrastructure segment revenues were $643 million for the quarter, 35% lower than 1Q 2020 due primarily due to reduced revenues from our industrial operations and a reduction in contributions from larger pipeline projects.
Despite the COVID-related headwinds, the segment delivered margins of 1.4%.
And although 170 basis points lower than 1Q 2020 primarily due to the reduced revenues, the results exceeded our original expectations for 1Q led by execution across much of our base business activity including our gas distribution and industrial services.
Our total backlog was a record $15.8 billion at the end of the first quarter with 12-month backlog of $8.9 billion representing solid increases when compared to year end as well as the first quarter of 2020.
For the first quarter of 2021, we generated free cash flow, a non-GAAP measure of $49 million, $115 million lower than 1Q 2020, however 1Q 2020 included the collection of $82 million of insurance proceeds associated with the settlement of two pipeline project claims.
Day sales outstanding or DSO measured 89 days for the first quarter, an increase of four days compared to the first quarter of 2020 and an increase of six days compared to December 31, 2020.
We had approximately $200 million of cash at the end of the quarter with total liquidity of approximately $2.1 billion and a debt-to-EBITDA ratio as calculated under our credit agreement of approximately 1.3 times.
Based on the Electric segment's strong first quarter and continued confidence in our ability to execute on the opportunities across the segment, we've increased the low end of our full year expectations for segment revenues resulting in a range between $8.4 billion and $8.5 billion for 2021.
Similarly, we are increasing the low end of our full year margin range for the segment with 2021 operating margins now expected to range between 10.2% and 10.9%.
Accordingly, we are reiterating our original full year guidance for the segment with revenues expected to range between $3.65 billion and $3.85 billion and segment margins ranging between 5.5% and 6%.
These segment operating ranges support our increased expectations for 2021 annual revenues of between $12.05 billion to $12.35 billion and adjusted EBITDA, a non-GAAP measure of between $1.1 billion and $1.2 billion.
The midpoint of the range represents 10% growth when compared to 2020's record adjusted EBITDA.
We now expect our full year tax rate to range between 25.25% and 25.75%.
As a result, our increased expectation for full year diluted earnings per share attributable to common stock is now between $3.25 and $3.69, and our increased expectation for adjusted diluted earnings per share attributable to common stock a non-GAAP measure is now between $4.12 and $4.57.
We are maintaining our free cash flow guidance for the year, expecting it to range between $400 million and $600 million.

Summary:
We believe this demonstrates the strength and sustainability of our business and long-term strategy, favorable end market trends, our ability to safely execute, and our strong competitive position in the marketplace.
Today we announced first quarter 2021 revenues of $2.7 billion.
Net income attributable to common stock was $90 million or $0.62 per diluted share and adjusted diluted earnings per share a non-GAAP measure was $0.83.
These segment operating ranges support our increased expectations for 2021 annual revenues of between $12.05 billion to $12.35 billion and adjusted EBITDA, a non-GAAP measure of between $1.1 billion and $1.2 billion.
As a result, our increased expectation for full year diluted earnings per share attributable to common stock is now between $3.25 and $3.69, and our increased expectation for adjusted diluted earnings per share attributable to common stock a non-GAAP measure is now between $4.12 and $4.57.