	December 31,
	2019		2018
$1.1 billion in Senior Notes, interest at 3.95% payable semiannually, principal due on December 1, 2042		1,088		1,088
$0.5 billion in Senior Notes, interest at 3.60% payable semiannually, principal due on December 1, 2029		493		—
$0.4 billion in Senior Notes, interest at 2.60% payable semiannually, principal due on December 1, 2022		399		1,394
Other debt		9		—
Long-term debt	$	1,989	$	2,482

Principal payments of debt for years subsequent to 2019 are as follows (in millions):

2020		—
2021		—
2022		400
2023		—
2024		—
Thereafter		1,589
	$	1,989

Effective October 30, 2019, the Company amended its five-year unsecured revolving credit facility, decreasing its borrowing availability to $2 billion and extended its maturity to October 30, 2024. The Company has the right to increase the aggregate commitments under this new agreement to an aggregate amount of up to $3.0 billion upon the consent of only those lenders holding any such increase. Interest under the new multicurrency facility is based upon LIBOR, NIBOR or CDOR plus 1.125% subject to a ratings-based grid or the U.S. prime rate. The new credit facility contains a financial covenant regarding maximum debt-to-capitalization ratio of 60%. As of December 31, 2019, the Company was in compliance with a debt-to-capitalization ratio of 22.4%.

The Company has a commercial paper program under which borrowings are classified as long-term since the program is supported by the $2.0 billion, five-year credit facility. At December 31, 2019, there were no commercial paper borrowings, and there were no outstanding letters of credit issued under the credit facility, resulting in $2.0 billion of funds available under this credit facility.

On November 4, 2019, the Company issued $500 million of 3.60% unsecured Senior Notes due 2029. The net proceeds were $493 million, after deducting $3 million in underwriting fees and a $4 million discount. Interest on each series of notes is due on June 1 and December 1 of each year, beginning on June 1, 2020. The Company may redeem some or all of the Senior Notes at any time at the applicable redemption price, plus accrued interest, if any, to the redemption date. At December 31, 2020, the Company was in compliance with the covenants under the indenture governing the Senior Notes.

On December 4, 2019, the Company repaid $1 billion of its 2.60% unsecured Senior Notes using available cash balances. Upon redemption, the Company paid $1,023 million, which included a redemption premium of $23.1 million as well as accrued and unpaid interest of $0.2 million. As a result of the redemption, the Company recorded a loss on extinguishment of debt of $26 million, which included the make whole premium of $23 million and non-cash charges of $3 million attributable to the write-off of unamortized discount and debt issuance costs.

The Company had $502 million of outstanding letters of credit at December 31, 2019, primarily in the U.S. and Norway, that are under various bilateral letter of credit facilities. Letters of credit are issued as bid bonds, advanced payment bonds and performance bonds.

At December 31, 2019 and 2018, the fair value of the Company’s unsecured Senior Notes approximated $1,947 million and $2,211 million, respectively. The fair value of the Company’s debt is estimated using Level 2 inputs in the fair value hierarchy and is based on quoted prices for those or similar instruments. At December 31, 2019 and 2018, the carrying value of the Company’s unsecured Senior Notes approximated $1,980 million and $2,482 million, respectively.