these 52 CRJ aircraft and through a reduction of our fleet size that was financed through leases as a result of scheduled lease expirations subsequent to September 30, 2018.
Special Items. The $21.9 million special items expense for the nine months ended September 30, 2019 related to a non-cash write-off of $18.5 million in aircraft manufacturer part credits that we forfeited to settle future lease return obligations with the aircraft manufacturer. The $18.5 million of expense was included in the SkyWest Airlines segment. The special items expense also included $3.4 million of expense associated with a cash payout of certain ExpressJet employees stock equity grants as part of the sale of ExpressJet, which was reflected in the ExpressJet segment.
Other operating expenses. Other operating expenses primarily consist of property taxes, hull and liability insurance, simulator costs, crew per diem, and crew hotel costs. The $21.9 million decrease in other operating expenses was primarily related to the sale of ExpressJet and the related decrease in crew costs associated with the decrease in expenses associated with that operation.
Interest Expense. The $10.4 million increase in interest expense was primarily related to the additional interest expense associated with the 13 E175 aircraft added to our fleet since September 30, 2018, which were debt financed.
Total airline expenses. The $213.6 million decrease in total airline expenses was primarily related to the sale of ExpressJet and the related expenses associated with ExpressJet’s prior operations, partially offset by additional operating expenses at SkyWest Airlines and SkyWest Leasing that resulted from new, additional aircraft we placed into service since September 30, 2018.
Summary of interest income, other income (expense) and provision for income taxes:
Interest income. Interest income increased $5.4 million, or 94.7%, during the nine months ended September 30, 2019, compared to the nine months ended September 30, 2018. The increase in interest income was primarily related to an increase in interest rates subsequent to September 30, 2018, and an increase in interest earned from loans to third parties executed subsequent to September 30, 2018.
Other income (expense), net. During the nine months ended September 30, 2019, we had other income, net of $43.9 million primarily related to the gain on sale of ExpressJet. During the nine months ended September 30, 2018, we had other income, net of $3.5 million primarily related to a mark-to-market gain on trading securities and excess rotable spare parts sold during the nine months ended September 30, 2018.
Income taxes. Our provision for income taxes was 23.2% and 22.6% for the nine months ended September 30, 2019 and 2018, respectively. The increase in the effective tax rate primarily relates to a smaller discrete tax benefit from excess tax deductions generated from employee equity transactions that occurred during the nine months ended September 30, 2019 compared to the nine months ended September 30, 2018, partially offset by a $1.5 million discrete tax benefit from the release of a valuation allowance on federal limited net operating losses that occurred during the nine months ended September 30, 2019.
Net income. Primarily due to the factors described above, we generated net income of $267.6 million, or $5.19 per diluted share, for the nine months ended September 30, 2019, compared to net income of $213.3 million, or $4.03 per diluted share, for the nine months ended September 30, 2018.
Our Business Segments
Three Months Ended September 30, 2019 and 2018
For the three months ended September 30, 2019 and following the sale of ExpressJet, we had two reportable segments, which were the basis of our internal financial reporting: SkyWest Airlines and SkyWest Leasing. Our segment disclosure relates to components of our business for which separate financial information is available to, and regularly evaluated by, our chief operating decision maker. Our operating segments for the three months ended September 30,