(in thousands)	2019			2018
Variable rate term loans¹	$	397,500		$	247,500
Fixed rate term loans²		296,000			296,000
7 1/2% Senior notes		—			150,000
Revenue bonds³		65,735			65,735
Medium-term notes⁴		3,000			3,000
Long-term principal		762,235			762,235
Interest rate swaps		—			(183	)
Debt issuance costs		(2,086	)		(2,143	)
Unamortized discounts		(3,680	)		(4,545	)
Total long-term debt		756,469			755,364
Less current portion of long-term debt		(45,974	)		(39,973	)
Long-term debt	$	710,495		$	715,391

Long-term debt at December 31, 2018 excludes $29.0 million of revenue bonds classified as held for sale as part of the sale of the Deltic MDF facility as they were assumed by the buyer. See Note 3: Sale of Deltic MDF Facility.

On March 22, 2018 we entered into a Second Amended and Restated Term Loan Agreement (Amended Term Loan Agreement), which amended the existing term loan agreement. The agreement included an additional $100.0 million of new term loans used to refinance Deltic’s $106.0 million credit facility and amended and restated a $100.0 million loan we assumed in connection with the Deltic merger to conform to the outstanding terms of the Amended Term Loan Agreement.

The $100.0 million repayment of Deltic’s credit facility was funded by a $100.0 million borrowing under our revolving credit facility and subsequently refinanced with two tranches of term loans aggregating $100.0 million under the Amended Term Loan Agreement.

In January 2019, through a first amendment to the Amended Term Loan Agreement, we refinanced $150.0 million of 7.5% senior notes (Senior Notes) that matured in 2019 with a $150.0 million term loan that matures in 2029. The new term loan carries a variable interest rate of one-month LIBOR plus 1.85%. We paid $0.5 million of lender fees on the new term loan. Concurrent with the new term loan, we entered into a $150.0 million interest rate swap to fix the rate at 4.56%. Upon refinancing, we redeemed and paid all outstanding Senior Notes including a redemption premium of $4.9 million which is included in the loss on extinguishment of debt in our Consolidated Statements of Income.

In December 2019, we refinanced an existing $40.0 million term loan that matured in December 2019 through a second amendment to the Amended Term Loan Agreement. The new term loan carries a variable interest rate of one-month LIBOR plus 1.85% and matures in 2029. In conjunction with the new term loan we entered into a $40.0 million interest rate swap to fix the rate at 3.17%. See Note: 15 Derivative Instruments.

At December 31, 2019 $693.5 million was outstanding under our Amended Term Loan Agreement. As of December 31, 2019, we were in compliance with all covenants under our debt agreements.

Debt issuance costs represent the capitalized direct costs incurred related to the issuance of debt. These costs are amortized to interest expense over the terms of the respective borrowings.

Unamortized discounts include a $4.9 million fair value adjustment to the $100.0 million term loan assumed in the Deltic merger. The unamortized balance of the fair value adjustment at December 31, 2019, was $3.7 million and will be amortized through the term loan’s maturity in 2025.

Scheduled principal payments due on long-term debt at December 31, 2019 are as follows:

(in thousands)
2020	$	46,000
2021		40,000
2022		43,000
2023		40,000
2024		175,735
Thereafter		417,500
Total	$	762,235

On February 14, 2018, we entered into a Second Amended and Restated Credit Agreement (Amended Credit Agreement) with an expiration date of April 13, 2023. The Amended Credit Agreement increased our revolving line of credit to $380.0 million, which may be increased by up to an additional $420.0 million. It also includes a sublimit of $75.0 million for the issuance of standby letters of credit and a sublimit of $25.0 million for swing line loans. Usage under either or both subfacilities reduces availability under the revolving line of credit.

We may also utilize borrowings under the Amended Credit Agreement to, among other things, refinance existing indebtedness and provide funding for working capital requirements, capital projects, acquisitions and other general corporate expenditures.

Pricing is set according to the type of borrowing. LIBOR Loans are issued at a rate equal to the LIBOR Rate, while Base Rate Loans are issued at a rate equal to a Base Rate, which is a fluctuating rate per annum equal to the highest of (a) the Federal Funds Rate plus one half of one percent, (b) the rate of interest in effect for such day as publicly announced from time to time by KeyBank as its prime rate and (c) the sum of the LIBOR rate that would apply to a one month Interest Period plus 1.00%. The interest rates we pay for borrowings under either type of loan include an additional Applicable Rate, which can range from 0.875% to 1.70% for LIBOR loans and from 0% to 0.70% for Base Rate loans, depending on our credit rating. As of December 31, 2019, we were able to borrow under the bank credit facility with an additional Applicable Rate of 1.30% for LIBOR Loans and 0.30% for Base Rate Loans, with facility fees of 0.20% on the $380.0 million of the bank credit facility.

The Amended Credit Agreement contains certain covenants that limit our ability and that of our subsidiaries to create liens, merge or consolidate, dispose of assets, incur indebtedness and guarantees, repurchase or redeem capital stock and indebtedness, make certain investments or acquisitions, enter into certain transactions with affiliates or change the nature of our business. The Amended Credit Agreement also contains financial maintenance covenants including the maintenance of a minimum interest coverage ratio and a maximum leverage ratio consistent with the Amended Term Loan Agreement. We are permitted to pay dividends to our stockholders under the terms of the Amended Credit Agreement so long as we expect to remain in compliance with the financial maintenance covenants. At December 31, 2019, we were in compliance with all covenants under our credit agreements, there were no borrowings under the revolving line of credit and approximately $1.0 million of the credit facility was utilized by outstanding letters of credit.