6.50% Senior Notes. In April 2020 and May 2020, we issued $300.0 million and $50.0 million, respectively, aggregate principal amounts of our 6.50% unsecured senior notes due May 1, 2025 at 100% and 101%, respectively, of the principal amounts (“6.50% Senior Notes”). The restrictions and covenants related to our 6.50% Senior Notes are similar to those governing our pre‑existing 4.625% unsecured senior notes due March 1, 2028 (“4.625% Senior Notes”). The unamortized amount of debt issuance costs relating to the 6.50% Senior Notes as of September 30, 2020 was $5.6 million. Interest expense, including amortization of debt issuance costs and debt premium, relating to the 6.50% Senior Notes was $6.0 million and $10.2 million for the quarter and nine months ended September 30, 2020, respectively. Interest accrues on the 6.50% Senior Notes beginning April 28, 2020 at a rate of 6.50% per annum and is payable semiannually on May 1 and November 1 of each year. The first interest payment date is November 1, 2020. The effective interest rate of the 6.50% Senior Notes is approximately 6.8% per annum, taking into account the amortization of premium and debt issuance costs.

The 6.50% Senior Notes were offered and sold in transactions not required to be registered under the Securities Act of 1933 and are not entitled to any registration rights. The fair value of the outstanding 6.50% Senior Notes, which are Level 1 liabilities calculated based on pricing from trades around the balance sheet date, was approximately $361.5 million at September 30, 2020.

The 6.50% Senior Notes are unsecured obligations and are guaranteed by each of our existing and future domestic subsidiaries that is a borrower or guarantor under the revolving credit facility (see Revolving Credit Facility below).

We may redeem the 6.50% Senior Notes at our option in whole or part at any time on or after May 1, 2022 at redemption prices (expressed as percentages of principal amount) of 103.250%, declining to 101.625% and 100.000% on or after May 1, 2023 and May 1, 2024, respectively, in each case plus any accrued and unpaid interest to the applicable redemption date. At any time prior to May 1, 2022, we may also redeem up to 40.0% of the 6.50% Senior Notes using the net proceeds from certain equity offerings at a redemption price equal to 106.500% of the principal amount plus any accrued and unpaid interest.

4.625% Senior Notes. In November 2019, we issued $500.0 million principal amount of 4.625% unsecured senior notes due March 1, 2028 at 100% of the principal amount. The unamortized amount of debt issuance costs as of September 30, 2020 was $6.7 million. Interest expense, including amortization of debt issuance costs, relating to the 4.625% Senior Notes was $6.0 million and $18.0 million for the quarter and nine months ended September 30, 2020, respectively. The effective interest rate of the 4.625% Senior Notes was approximately 4.8% per annum, taking into account the amortization of debt issuance costs. The fair value of the

outstanding 4.625% Senior Notes, which are Level 1 liabilities, was approximately $468.3 million and $513.5 million at September 30, 2020 and December 31, 2019, respectively.

5.875% Senior Notes. In May 2016, we issued $375.0 million principal amount of 5.875% unsecured senior notes due May 15, 2024 (“5.875% Senior Notes”) at 100% of the principal amount. On December 18, 2019, we redeemed in full all outstanding 5.875% Senior Notes at a redemption price of 104.406% of the principal amount. The effective interest rate of the 5.875% Senior Notes was approximately 6.1% per annum, taking into account the amortization of debt issuance costs. Interest expense, including amortization of debt issuance costs, relating to the 5.875% Senior Notes was $5.7 million and $17.1 million for the quarter and nine months ended September 30, 2019, respectively.

The amount of interest expense capitalized as construction in progress was $0.3 million and $0.4 million during the quarters ended September 30, 2020 and September 30, 2019, respectively. The amount of interest expense capitalized as construction in progress was $0.8 million and $1.3 million during the nine months ended September 30, 2020 and September 30, 2019, respectively.

Our credit agreement with Wells Fargo Bank, National Association, as administrative agent, and the other financial institutions party thereto (“Revolving Credit Facility”) provides us with a $375.0 million funding commitment through October 2024.

The table below summarizes availability and usage of our Revolving Credit Facility as determined by a borrowing base calculated as of September 30, 2020 (in millions of dollars except for borrowing rate):

Revolving Credit Facility borrowing commitment	$	375.0
Borrowing base availability	$	261.0
Less: Outstanding borrowings under Revolving Credit Facility		—
Less: Outstanding letters of credit under Revolving Credit Facility		(7.7	)
Remaining borrowing availability	$	253.3
Borrowing rate (if applicable)¹		3.50	%