Turning to Slide 5, I am pleased to report improved earnings for both the fourth quarter and full year 2011. In the fourth quarter of 2011, we posted a 23% increase in diluted earnings per share adjusted over the same period in 2010. And for the full year, we posted about an 8% increase over the full year 2010. These favorable results are from the solid execution of our business plan.
As for 2012, we are introducing guidance for diluted earnings per share of $3.35 to $3.55 with a midpoint of $3.45. Joe will have more to say on that in just a few moments.
Slide 6 provides a brief operational update on our regulated utilities. Our utilities finished the year strong. As shown on Slide 19 in the appendix, the actual returns on equity for our natural gas and electric utilities were about 8% and 11% respectively in 2011. We expect some slight improvement in 2012, as our returns on equity for our utilities are projected to be in the range of 9% to 10%.
In terms of net income the shortfall of our actual utility earnings to authorized level was about $15.4 million in 2011, which is much better than $37 million that we had estimated at this time last year. The progress we made in 2011 places us within a range of what we can reasonably expect on a continuing basis.From a rate perspective we reached resolutions on four of the five rate cases that were pending in 2011. This included rate cases for Peoples Gas, North Shore Gas and Upper Peninsula Power as well as the Wisconsin Public Service limited reopener case. The loan case that did not come to a final decision in 2011 was Minnesota Energy Resources, but interim rates were granted in February of 2011. The key details for each rate case are summarized in appendix Slides 22 to 26.