The following table compares our other consolidated operating results for 2019 and 2018:
Interest expense. Interest expense increased to $279.1 million in 2019, compared to $254.1 million in 2018, primarily due to: • an increase of $29.9 million primarily to the commencement of Teekay LNG's finance lease obligations upon the deliveries of the Myrina, Megara and Yamal Spirit LNG carriers and an increase in debt balance to pay for the final newbuilding installments on the Bahrain Spirit and Sean Spirit LNG carrier newbuilding deliveries;
• an increase of $6.7 million primarily due to the additional interest expense incurred by Teekay Tankers with respect to three sale-leaseback financing transactions completed in September 2018, November 2018 and May 2019; and • an increase of $6.3 million due to decreases in capitalized interest in Teekay LNG in 2019 as a result of vessels delivered during 2018 and 2019;
• an increase of $6.7 million primarily due to the additional interest expense incurred by Teekay Tankers with respect to three sale-leaseback financing transactions completed in September 2018, November 2018 and May 2019; and • an increase of $6.3 million due to decreases in capitalized interest in Teekay LNG in 2019 as a result of vessels delivered during 2018 and 2019;
• a decrease of $4.1 million relating to interest incurred by Teekay Parent in 2018 as a result of the prepayment of the outstanding amounts under one revolving credit facility and lower debt issuance cost amortization in 2019 on an amendment of another revolving credit facility.
Realized and unrealized (losses) gains on non-designated derivative instruments. Realized and unrealized (losses) gains related to derivative instruments that are not designated as hedges for accounting purposes are included as a separate line item in the consolidated statements of loss. Net realized and unrealized losses on non-designated derivatives were $13.7 million for 2019, compared to $14.9 million for 2018, as detailed in the table below:
The realized losses relate to amounts we actually realized for settlements related to these derivative instruments in normal course and amounts
paid to terminate interest rate swap agreement terminations.
During 2019 and 2018, we had interest rate swap agreements with aggregate average net outstanding notional amounts of approximately $1.1 billion and $1.3 billion, respectively, with average fixed rates of approximately 3.0% and 2.9%, respectively. Short-term variable benchmark interest rates during these periods were generally less than 3.0% and, as such, we incurred realized losses of $8.3 million and $13.9 million during 2019 and 2018, respectively, under the interest rate swap agreements.
We did not incur any realized losses related to the termination of interest rate swaps in 2019, compared to realized losses of $13.7 million during 2018. Primarily as a result of significant changes in long-term benchmark interest rates during 2019 and 2018, we recognized unrealized losses of $7.9 million in 2019 compared to unrealized gains of $33.7 million in 2018 under the interest rate swap agreements.
During the year ended December 31, 2019, we recognized a reversal of previously unrealized losses of $26.9 million on all the warrants held by Teekay to purchase common units of Altera (or the Warrants) as a result of the sale of the Warrants to Brookfield, and we concurrently recognized a realized loss of $25.6 million during the same period. During the year ended December 31, 2018, we recognized unrealized losses of $21.1 million on the Warrants. Please read “Item 18 – Financial Statements: Note 12 – Fair Value Measurements and Financial Instruments.”
Foreign Exchange Loss. Foreign currency exchange losses were $13.6 million in 2019 compared to gains of $6.1 million in 2018. Our foreign currency exchange gains and losses, substantially all of which are unrealized, are primarily due to the relevant period-end revaluation of our Norwegian-Krone (or NOK)-denominated debt and our Euro-denominated term loans, finance leases and restricted cash for financial reporting purposes and the realized and unrealized (losses) gains on our cross currency swaps.
Gains on NOK-denominated and Euro-denominated monetary liabilities reflect a stronger U.S. Dollar against the NOK and Euro on the date of revaluation or settlement compared to the rate in effect at the beginning of the period. Losses on NOK-denominated and Euro-denominated monetary liabilities reflect a weaker U.S. Dollar against the NOK and Euro on the date of revaluation or settlement compared to the rate in effect at the beginning of the period.
For 2019, foreign currency exchange loss included realized losses of $5.1 million (2018 – $6.5 million) and unrealized losses of $13.2 million (2018 – gains of $21.2 million) on our cross currency swaps, realized losses on maturity and termination of cross currency swaps of $nil (2018 – $42.3 million) and unrealized gains of $5.8 million (2018 – gains of $19.2 million) on the revaluation of our NOK-denominated debt.
Loss on deconsolidation of Altera. Loss on deconsolidation of Altera was $7.1 million in 2018. Please read "Item 18 – Financial Statements: Note 4 – Deconsolidation and Sale of Altera."
Other loss. Other loss was $14.5 million in 2019 compared to $2.0 million in 2018. Other loss in 2019 was primarily due to losses of $10.6 million on the repurchase of 2020 Notes, $2.0 million tax expense on Teekay LNG's income from U.S. sources and $1.4 million losses relating to Teekay LNG's sale lease-back refinancing of the Torben Spirit completed in 2019. Other loss in 2018 included $1.8 million related to repurchases of the 2020 Notes and $0.6 million related to the tax indemnification guarantee liability related to the Teekay Nakilat finance lease.
Income Tax Expense. Income tax expense was $25.5 million in 2019 compared to $19.7 million in 2018. This increase in income tax expense was primarily due to changes to freight tax accruals in 2019.
Table:
 | Year Ended December 31, | 
(in thousands of U.S. dollars, except percentages) | 2019 | 2018
Interest expense | (279,059) | (254,126)
Interest income | 7,804 | 8,525
Realized and unrealized losses on non-designated derivative instruments | (13,719) | (14,852)
Foreign exchange (loss) gain | (13,574) | 6,140
Loss on deconsolidation of Altera | — | (7,070)
Other loss | (14,475) | (2,013)
Income tax expense | (25,482) | (19,724)


Question: What led to increase in income tax expense in 2019?
Answer: primarily due to changes to freight tax accruals in 2019.