11. Debt and lines of credit

Short-term borrowings

We maintain a line of credit to support commercial paper borrowings, if any, and to provide additional liquidity through bank loans. As of December 31, 2017, we had a variable-rate revolving credit facility from a consortium of investment-grade banks that allows us to borrow up to $2 billion until March 2022. The interest rate on borrowings under this credit facility, if drawn, is indexed to the applicable London Interbank Offered Rate (LIBOR). As of December 31, 2017, our credit facility was undrawn and we had no commercial paper outstanding.

Long-term debt

We retired $250 million of maturing debt in March 2017 and another $375 million in June 2017.

In May 2017, we issued an aggregate principal amount of $600 million of fixed-rate, long-term debt. The offering consisted of the reissuance of $300 million of 2.75% notes due in 2021 at a premium and the issuance of $300 million of 2.625% notes due in 2024 at a discount. We incurred $3 million of issuance and other related costs. The proceeds of the offerings were $605 million, net of the original issuance discount and premium, and were used for the repayment of maturing debt and general corporate purposes.

In November 2017, we issued a principal amount of $500 million of fixed-rate, long-term debt due in 2027. We incurred $3 million of issuance and other related costs. The proceeds of the offering were $494 million, net of the original issuance discount, and were used for general corporate purposes.

In May 2016, we issued a principal amount of $500 million of fixed-rate, long-term debt due in 2022. We incurred $3 million of issuance and other related costs. The proceeds of the offering were $499 million, net of the original issuance discount, and were used toward the repayment of a portion of $1.0 billion of maturing debt retired in May 2016.

In May 2015, we issued a principal amount of $500 million of fixed-rate, long-term debt due in 2020. We incurred $3 million of issuance and other related costs. The proceeds of the offering were $498 million, net of the original issuance discount, and were used toward the repayment of a portion of the debt that matured in August 2015. We retired $250 million of maturing debt in April 2015 and another $750 million in August 2015.

Long-term debt outstanding is as follows:

	December 31,
	2017			2016
Notes due 2017 at 0.875%	$	—		$	250
Notes due 2017 at 6.60% (assumed with National acquisition)		—			375
Notes due 2018 at 1.00%		500			500
Notes due 2019 at 1.65%		750			750
Notes due 2020 at 1.75%		500			500
Notes due 2021 at 2.75%		550			250
Notes due 2022 at 1.85%		500			500
Notes due 2023 at 2.25%		500			500
Notes due 2024 at 2.625%		300			—
Notes due 2027 at 2.90%		500			—
Total debt		4,100			3,625
Net unamortized discounts, premiums and debt issuance costs		(23	)		(16	)
Total debt, including net unamortized discounts, premiums and debt issuance costs		4,077			3,609
Current portion of long-term debt		(500	)		(631	)
Long-term debt	$	3,577		$	2,978

Interest and debt expense was $78 million in 2017, $80 million in 2016 and $90 million in 2015. This was net of the amortization of the debt discounts, premiums and debt issuance costs. Cash payments for interest on long-term debt were $75 million in 2017, $88 million in 2016 and $99 million in 2015. Capitalized interest was not material.