Welcome to Assurant's First Quarter 2018 Earnings Conference Call and Webcast. (Operator Instruction) It is now my pleasure to turn the floor over to Suzanne Shepherd, Vice President of Investor Relations. You may begin.
Thank you, Christina, and good morning, everyone.
We look forward to discussing our first quarter 2018 results with you today. Joining me for Assurant's conference call are Alan Colbert, our President and Chief Executive officer; and Richard Dziadzio, our Chief Financial Officer and Treasurer.
Yesterday, after the market closed, we issued a news release announcing our results for the first quarter of 2018. The release and corresponding financial supplement are available on assurant.com. We'll start today's call with brief remarks from Alan and Richard before moving into a Q&A session. Some of the statements made today may be forward-looking. Forward-looking statements are subject to risks and uncertainties and other factors that may cause actual results to differ materially from those contemplated by these statements. Additional information regarding these factors can be found in yesterday's earnings news release as well as in our SEC reports.
During the first quarter, we completed the financing related to our acquisition of The Warranty Group or TWG. This included a $1.3 billion dollar debt issuance of which $350 million were placed debt that matured during the quarter and a $288 million mandatory convertible preferred stock issuance. The acquisition is expected to close in the second quarter subject to regulatory approval and other customary closing conditions. As such, our first quarter 2018 results and full year 2018 outlook do not include any contribution from The Warranty Group nor the impact of the $1.2 billion of acquisition related financing.
Prior to closing, net operating income and net operating income per diluted share will exclude the interest expense and dilutive impact of the mandatory convertible preferred stock issued in connection with the acquisition financing. These amounts, however, will be reflected in our GAAP results. Once the transaction closes, we will include the related interest expense and dilutive impact of the shares in operating results using the if-converted accounting methodology.
During today's call, we will also refer to other non-GAAP financial measures which we believe are important in evaluating the Company's performance. For more details on these non-GAAP measures, the most comparable GAAP measures and a reconciliation of the two, please refer yesterday's news release and financial supplements available on assurant.com.
I will now turn the call over to Alan.
Good morning, everyone. Overall, we are pleased with our first quarter performance. Results were in line with our expectations. A lower effective tax rate following the enactment of US tax reform was the primary driver of the earnings increase as well as modest underlying business growth. This growth was partially offset by increased corporate expenses and higher catastrophe losses.
We also continue to make good progress preparing for the closing of The Warranty Group acquisition. This included submitting the required filings and raising the necessary funds to support the acquisition. Integration planning efforts continue