Debt

Debt consists of (in millions):


	March 31, 2018		December 31, 2017
$1.4 billion in Senior Notes, interest at 2.60% payable semiannually, principal due on December 1, 2022	$	1,393	$	1,392
$1.1 billion in Senior Notes, interest at 3.95% payable semiannually, principal due on December 1, 2042		1,088		1,088
Other		232		232

Total debt		2,713		2,712
Less current portion		6		6

Long-term debt	$	2,707	$	2,706

The Company has a $3.0 billion, five-year unsecured revolving credit facility, which expires on June 27, 2022. The Company has the right to increase the aggregate commitments under this agreement to an aggregate amount of up to $4.0 billion upon the consent of only those lenders holding any such increase. Interest under the 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 credit facility contains a financial covenant regarding maximum debt-to-capitalization ratio of 60%. As of March 31, 2018, the Company was in compliance with a debt-to-capitalization ratio of 16.1%.

The Company has a commercial paper program under which borrowings are classified as long-term since the program is supported by the $3.0 billion, five-year credit facility. At March 31, 2018, there were no commercial paper borrowings, and there were no outstanding letters of credit issued under the credit facility, resulting in $3.0 billion of funds available under this credit facility.

The Company had $567 million of outstanding letters of credit at March 31, 2018, primarily in the U.S. and Norway, that are under various bilateral committed letter of credit facilities. Letters of credit are issued as bid bonds, advanced payment bonds and performance bonds.

At March 31, 2018 and December 31, 2017, the fair value of the Company’s unsecured Senior Notes approximated $2,323 million and $2,346 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 March 31, 2018 and December 31, 2017, the carrying value of the Company’s unsecured Senior Notes approximated $2,481 million and $2,480 million, respectively.