Debt

A summary of long-term debt outstanding is as follows:



(in millions)	March 31, 2019		December 31, 2018
3.3% Senior Notes, due 2020 ¹	698		698
4.0% Senior Notes, due 2025 ²	693		693
4.4% Senior Notes, due 2026 ³	893		892
2.95% Senior Notes, due 2027 ⁴	493		493
6.55% Senior Notes, due 2037 ⁵	396		396
4.5% Senior Notes, due 2048 ⁶	490		490
Long-term debt	$	3,663	$	3,662


¹	Interest payments are due semiannually on February 14 and August 14, and as of March 31, 2019, the unamortized debt discount and issuance costs total $2 million.


²	Interest payments are due semiannually on June 15 and December 15, and as of March 31, 2019, the unamortized debt discount and issuance costs total $7 million.


³	Interest payments are due semiannually on February 15 and August 15, and as of March 31, 2019, the unamortized debt discount and issuance costs total $7 million.


⁴	Interest payments are due semiannually on January 22 and July 22, and as of March 31, 2019, the unamortized debt discount and issuance costs total $7 million.


⁵	Interest payments are due semiannually on May 15 and November 15, and as of March 31, 2019, the unamortized debt discount and issuance costs total $4 million.


⁶	Interest payments are due semiannually on May 15 and November 15, beginning on November 15, 2018, and as of March 31, 2019, the unamortized debt discount and issuance costs total $10 million.

The fair value of our long-term debt borrowings was $4.0 billion and $3.8 billion as of March 31, 2019 and December 31, 2018, respectively, and was estimated based on quoted market prices.

We have the ability to borrow a total of $1.2 billion through our commercial paper program, which is supported by our revolving $1.2 billion five-year credit agreement (our "credit facility") that we entered into on June 30, 2017. This credit facility will terminate on June 30, 2022. As of March 31, 2019 and December 31, 2018, there were no commercial paper borrowings outstanding.

Depending on our corporate credit rating, we pay a commitment fee of 8 to 17.5 basis points for our credit facility, whether or not amounts have been borrowed. We currently pay a commitment fee of 10 basis points. The interest rate on borrowings under our credit facility is, at our option, calculated using rates that are primarily based on either the prevailing London Inter-Bank Offer Rate, the prime rate determined by the administrative agent or the Federal Funds Rate. For certain borrowings under this credit facility, there is also a spread based on our corporate credit rating.

Our credit facility contains certain covenants. The only financial covenant requires that our indebtedness to cash flow ratio, as defined in our credit facility, is not greater than 4 to 1, and this covenant level has never been exceeded.