9. Debt

Debt consists of (in millions):


	December 31,
	2015		2014
Senior Notes, interest at 6.125% payable semiannually, principal due on August 15, 2015		—		151
Senior Notes, interest at 1.35% payable semiannually, principal due on December 1, 2017		500		500
Senior Notes, interest at 2.6% payable semiannually, principal due on December 1, 2022		1,396		1,396
Senior Notes, interest at 3.95% payable semiannually, principal due on December 1, 2042		1,096		1,096
Commercial paper		893		—
Other		45		23

Total debt		3,930		3,166
Less current portion		2		152

Long-term debt	$	3,928	$	3,014

Principal payments of debt for years subsequent to 2015 are as follows (in millions):


2016	$	2
2017		506
2018		898
2019		5
2020		5
Thereafter		2,514

	$	3,930

On August 15, 2015, the Company repaid $151 million of its 6.125% unsecured Senior Notes using available cash balances.

During the second quarter of 2015, the Company exercised its accordion option to increase aggregate borrowing capacity under its five-year unsecured revolving credit facility by an additional $1.0 billion, bringing the aggregate borrowing capacity to $4.5 billion. The facility expires September 28, 2018. The Company also has a commercial paper program under which borrowings are classified as long-term since the program is supported by the $4.5 billion, five-year unsecured revolving credit facility. At December 31, 2015, there were $893 million in commercial paper borrowings, and there were no outstanding letters of credit issued under the credit facility, resulting in $3,607 million of funds available under this revolving credit facility. Interest under this multicurrency facility is based upon LIBOR, NIBOR or EURIBOR plus 0.875% subject to a ratings-based grid, or the U.S. prime rate. The credit facility contains a financial covenant regarding maximum debt to capitalization and the Company was in compliance at December 31, 2015.

The Company also had $2,378 million of additional outstanding letters of credit at December 31, 2015, primarily in the U.S., that are under various bilateral committed letter of credit facilities. Other letters of credit are issued as bid bonds, advanced payment bonds and performance bonds.

At December 31, 2015 and 2014, the fair value of the Company’s unsecured Senior Notes approximated $2,551 million and $2,974 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, 2015 and 2014, the carrying value of the Company’s unsecured Senior Notes approximated $2,992 million and $3,143 million, respectively.