Debt

9 Months Ended

Sep. 30, 2020

Debt Disclosure [Abstract]

Debt

Note 7—Debt

	Millions of Dollars
	September 30 2020		December 31 2019
2.450% Senior Notes due December 2024	$	300	300
3.605% Senior Notes due February 2025	500		500
3.550% Senior Notes due October 2026	500		500
3.750% Senior Notes due March 2028	500		500
3.150% Senior Notes due December 2029	600		600
4.680% Senior Notes due February 2045	450		450
4.900% Senior Notes due October 2046	625		625
Tax-exempt bonds due April 2020 and April 2021 at 0.535% and 1.85% at September 30, 2020, and December 31, 2019, respectively	50		75
Revolving credit facility borrowings due October 2020 at 1.401%	290		—
Debt at face value	3,815		3,550
Net unamortized discounts and debt issuance costs	(32)		(34)
Total debt	3,783		3,516
Short-term debt	(340)		(25)
Long-term debt	$	3,443	3,491

The fair value of our fixed-rate and floating-rate debt is estimated based on observable market prices and is classified as Level 2 of the fair value hierarchy. The fair value of our fixed-rate debt was $3,567 million and $3,650 million at September 30, 2020, and December 31, 2019, respectively. The fair value of our floating-rate debt approximated carrying value of $340 million and $75 million at September 30, 2020, and December 31, 2019, respectively.

At September 30, 2020, borrowings of $290 million were outstanding and $3 million in letters of credit had been drawn under our $750 million revolving credit facility.

On April 1, 2020, we repaid the $25 million tranche of tax-exempt bonds due April 2020. The two remaining tranches, totaling $50 million, mature in April 2021.