Debt

The following table summarizes the carrying value of our outstanding debt:



	Coupon		As of			Effective		As of			Effective
	Rate		March 31, 2016			Interest Rate		December 31, 2015			Interest Rate
	(In millions, except percentages)
Long-Term Debt
Floating Rate Notes:
Senior notes due 2017	LIBOR plus 0.20%		$	450		0.651	%	$	450		0.586	%
Senior notes due 2019	LIBOR plus 0.48%		400			0.853	%	400			0.825	%

Fixed Rate Notes:
Senior notes due 2017	1.350	%	1,000			1.456	%	1,000			1.456	%
Senior notes due 2018	2.500	%	750			2.775	%	—			—	%
Senior notes due 2019	2.200	%	1,150			2.346	%	1,150			2.346	%
Senior notes due 2020	3.250	%	500			3.389	%	500			3.389	%
Senior notes due 2021	2.875	%	750			2.993	%	750			2.993	%
Senior notes due 2022	3.800	%	750			3.989	%	—			—	%
Senior notes due 2022	2.600	%	1,000			2.678	%	1,000			2.678	%
Senior notes due 2024	3.450	%	750			3.531	%	750			3.531	%
Senior notes due 2042	4.000	%	750			4.114	%	750			4.114	%
Senior notes due 2056	6.000	%	750			6.547	%	—			—	%
Total senior notes			9,000					6,750
Hedge accounting fair value adjustments			104					41
Unamortized discount and debt issuance costs			(74		)			(42		)
Total long-term debt			$	9,030				$	6,749

Short-Term Debt
Other indebtedness			6					—
Total short-term debt			6					—
Total Debt			$	9,036				$	6,749

Senior Notes

In the three months ended March 31, 2016, we issued senior unsecured notes, or senior notes, in an aggregate principal amount of $2.25 billion. This consists of $750 million aggregate principal amount of 2.500% fixed rate notes due 2018, $750 million aggregate principal amount of 3.800% fixed rate notes due 2022 and $750 million aggregate principal amount of 6.000% fixed rate notes due 2056.

The floating rate notes are not redeemable prior to maturity. On and after March 1, 2021, we may redeem some or all of the fixed rate notes due 2056 at any time and from time to time prior to their maturity at a redemption price equal to 100% of the principal amount of the notes to be redeemed, plus accrued and unpaid interest. We may redeem some or all of the other fixed rate notes of each series at any time and from time to time prior to their maturity, generally at a make-whole redemption price, plus accrued and unpaid interest. If a change of control triggering event (as defined in the applicable notes) occurs with respect to the 2.500% fixed rate notes due 2018, the 3.800% fixed rate notes due 2022 or the 6.000% fixed rate notes due 2056, we must, subject to certain exceptions, offer to repurchase all of the notes of the applicable series at a price equal to 101% of the principal amount, plus accrued and unpaid interest.

To help achieve our interest rate risk management objectives, in connection with the previous issuance of certain senior notes, we entered into interest rate swap agreements that effectively converted $2.4 billion of our fixed rate debt to floating rate debt based on LIBOR plus a spread. These swaps were designated as fair value hedges against changes in the fair value of certain fixed rate senior notes resulting from changes in interest rates. The gains and losses related to changes in the fair value of interest rate swaps substantially offset changes in the fair value of the hedged portion of the underlying debt that are attributable to changes in market interest rates.

The effective interest rates for our senior notes include the interest payable, the amortization of debt issuance costs and the amortization of any original issue discount on these senior notes. Interest on these senior notes is payable either quarterly or semiannually. Interest expense associated with these senior notes, including amortization of debt issuance costs, during the three months ended March 31, 2016 and 2015 was approximately $50 million and $45 million, respectively. At March 31, 2016, the estimated fair value of these senior notes was approximately $8.9 billion.

The indenture pursuant to which the senior notes were issued includes customary covenants that, among other things and subject to exceptions, limit our ability to incur, assume or guarantee debt secured by liens on specified assets or enter into sale and lease-back transactions with respect to specified properties, and also includes customary events of default.

Commercial Paper

We have an up to $1.5 billion commercial paper program pursuant to which we may issue commercial paper notes with maturities of up to 397 days from the date of issue in an aggregate principal amount at maturity of up to $1.5 billion outstanding at any time. As of March 31, 2016, there were no commercial paper notes outstanding.

Credit Agreement

As of March 31, 2016, no borrowings were outstanding under our $2 billion credit agreement. However, as described above, we have an up to $1.5 billion commercial paper program and therefore maintain $1.5 billion of available borrowing capacity under our credit agreement in order to repay commercial paper borrowings in the event we are unable to repay those borrowings from other sources when they become due. As a result, at March 31, 2016, $500 million of borrowing capacity was available for other purposes permitted by the credit agreement. The credit agreement includes customary representations, warranties, affirmative and negative covenants, including financial covenants, events of default and indemnification provisions in favor of the banks. The negative covenants include restrictions regarding the incurrence of liens and subsidiary indebtedness, in each case, subject to certain exceptions. The financial covenants require us to meet a quarterly financial test with respect to a minimum consolidated interest coverage ratio and a maximum consolidated leverage ratio. The events of default include the occurrence of a change of control (as defined in the credit agreement) with respect to us.

We were in compliance with all covenants in our outstanding debt instruments for the three-month period ended March 31, 2016.