DECEMBER 31	2018			2017
(in millions)
Senior debentures maturing April 15, 2026	$	375.0		$	375.0
Senior debentures maturing October 15, 2025		62.6			62.6
Subordinated debentures maturing March 30, 2053		175.0			175.0
Subordinated debentures maturing February 3, 2027		50.1			59.7
FHLB borrowings (secured)		125.0			125.0
Total principal debt		787.7			797.3
Unamortized debt issuance costs		(9.8	)		(10.4	)
Total	$	777.9		$	786.9

The Company has outstanding 7.625% unsecured senior debentures with a par value of $62.6 million as of December 31, 2018 and 2017 and mature on October 15, 2025. Additionally, the Company has outstanding unsecured senior debentures that were issued on April 8, 2016 with a par value of $375.0 million, as of December 31, 2018 and 2017 and mature on April 15, 2026. Both 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 has outstanding $175.0 million aggregate principal amount of 6.35% subordinated unsecured debentures due March 30, 2053. These debentures pay interest quarterly. The Company may redeem these debentures in whole at any time, or in part from time to time, on or after March 30, 2018, at a redemption price equal to their principal amount plus accrued and unpaid interest. In addition, the Company’s subordinated debentures maturing February 3, 2027 have a par value of $50.1 million as of December 31, 2018 and $59.7 million as of December 31, 2017 and pay cumulative dividends semi-annually at 8.207%.

During the third quarter of 2018, the Company repurchased subordinated debentures maturing February 3, 2027 with a net carrying value of $9.6 million at a cost of $11.5 million, resulting in a loss of $1.9 million.

In 2009, Hanover Insurance received a $125.0 million advance through its membership in the FHLB. This collateralized advance bears interest at a fixed rate of 5.50% per annum over a twenty-year term. As collateral to FHLB, the Company pledged government agency securities with a fair value of $234.7 million and $223.6 million, for the aggregate borrowings of $125.0 million as of December 31, 2018 and December 31, 2017, respectively. The fair value of the collateral pledged must be maintained at certain specified levels of the borrowed amount, which can vary depending on the type of assets pledged. If the fair value of this collateral declines below these specified levels, the Company would be required to pledge additional collateral or repay outstanding borrowings. The Company is permitted to voluntarily repay the outstanding borrowings at any time, subject to a repayment fee. As a requirement of membership in the FHLB, the Company maintains a certain level of investment in FHLB stock. Total holdings of FHLB stock were $8.7 million and $8.5 million at December 31, 2018 and 2017, respectively. At December 31, 2018, the Company notified FHLB of its intent to repay its $125 million advance and recorded a pre-tax charge of $26.3 million related to the pre-payment provision.

At December 31, 2018, the Company had a $200.0 million credit agreement which expires in May 2019. The Company had no borrowings under this agreement as of December 31, 2018.

Interest expense was $45.1 million, $45.2 million, and $51.4 million in 2018, 2017 and 2016, respectively. At December 31, 2018, the Company was in compliance with the covenants associated with all of its debt indentures and credit arrangements.