DECEMBER 31	2020			2019
(in millions)
Senior debentures maturing April 15, 2026	$	375.0		$	375.0
Senior debentures maturing September 1, 2030		300.0			—
Senior debentures maturing October 15, 2025		61.8			62.6
Subordinated debentures maturing February 3, 2027		50.1			50.1
Subordinated debentures maturing March 30, 2053		—			175.0
Total principal debt		786.9			662.7
Unamortized debt issuance costs		(6.1	)		(9.3	)
Total	$	780.8		$	653.4

On August 24, 2020, the Company issued $300.0 million aggregate principal amount of 2.50% unsecured senior debentures maturing September 1, 2030. Net proceeds from the issuance were $296.4 million. On September 19, 2020, the Company used a portion of these net proceeds to redeem its outstanding $175.0 million principal amount of 6.35% subordinated unsecured debentures due March 30, 2053. The Company recognized $6.1 million of unamortized debt issuance costs associated with this debt.

The Company also held $375.0 million par value of unsecured senior debentures at December 31, 2020 and 2019, that were issued on April 8, 2016, and mature on April 15, 2026. Additionally, the Company had outstanding 7.625% unsecured senior debentures with a par value of $61.8 million and $62.6 million as of December 31, 2020 and 2019, respectively, maturing on October 15, 2025. All of the Company’s outstanding senior debentures are subject to certain restrictive covenants, including limitations on the issuance or disposition of stock of restricted subsidiaries and limitations on liens, and pay interest semi-annually.

The Company also held subordinated debentures maturing February 3, 2027 which had a par value of $50.1 million as of December 31, 2020 and 2019, and pay cumulative dividends semi-annually at 8.207%.

On January 2, 2019, the Company repaid $125.0 million of its FHLB advances that were due 2029 and had an interest rate of 5.5%, along with related prepayment fees of $26 million; such fees were recognized in 2018. Membership in FHLB provides the Company with access to additional short-term liquidity based on the level of investment in FHLB stock and pledged collateral. Total holdings of FHLB stock were $2.6 million and $2.1 million at December 31, 2020 and 2019, respectively. At December 31, 2020 and 2019, the Company had pledged government agency securities with a fair value of $103.9 million and $94.0 million, respectively, as collateral for periodic short-term borrowings with the FHLB. There were no borrowings outstanding with the FHLB at December 31, 2020 or 2019.

At December 31, 2020, the Company had a $200.0 million credit agreement which expires in April 2024. The Company had no borrowings under this agreement as of December 31, 2020.

Interest expense was $37.1 million, $37.5 million, and $45.1 million in 2020, 2019 and 2018, respectively. At December 31, 2020, the Company was in compliance with the covenants associated with all of its debt indentures and credit arrangements.