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 unamortized amount of debt issuance costs relating to the 6.50% Senior Notes as of March 31, 2021 was

$5.0 million. Interest expense, including amortization of debt issuance costs and debt premium, relating to the 6.50% Senior Notes was $6.0 million for the quarter ended March 31, 2021. 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 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 $371.3 million and $376.8 million at March 31, 2021 and December 31, 2020, respectively.

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 (“4.625% Senior Notes”). The unamortized amount of debt issuance costs as of March 31, 2021 was $6.3 million. Interest expense, including amortization of debt issuance costs, relating to the 4.625% Senior Notes was $6.0 million for both quarters ended March 31, 2021 and March 31, 2020. 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 $511.7 million and $521.3 million at March 31, 2021 and December 31, 2020, respectively.

The amount of interest expense capitalized as construction in progress was $0.1 million and $0.3 million during the quarters ended March 31, 2021 and March 31, 2020, 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 following table summarizes availability and usage of our Revolving Credit Facility as determined by a borrowing base calculated as of March 31, 2021 (in millions of dollars except for borrowing rate):

Revolving Credit Facility borrowing commitment	$	375.0
Borrowing base availability	$	375.0
Less: Outstanding borrowings under Revolving Credit Facility		—
Less: Outstanding letters of credit under Revolving Credit Facility		(7.8	)
Remaining borrowing availability	$	367.2
Borrowing rate (if applicable)¹		3.50	%