The components of long-term debt (net of discounts), along with maturity dates for the years subsequent to May 31, 2015, are as follows (in millions):

				May 31,
				2015		2014
Senior unsecured debt:
	Interest Rate %		Maturity
	8.00		2019		$ 750		$ 750
	2.30		2020		399		-
	2.625-2.70		2023		749		748
	4.00		2024		749		749
	3.20		2025		699		-
	4.90		2034		499		499
	3.90		2035		498		-
	3.875-4.10		2043		992		992
	5.10		2044		749		749
	4.10		2045		646		-
	4.50		2065		248		-
	7.60		2098		239		239
Total senior unsecured debt					7,217		4,726
Capital lease obligations					51		11
					7,268		4,737
		Less current portion			19		1
				$	7,249	$	4,736

Interest on our fixed-rate notes is paid semi-annually. Long-term debt, exclusive of capital leases, had estimated fair values of $7.4 billion at May 31, 2015 and $5.0 billion at May 31, 2014. The estimated fair values were determined based on quoted market prices and the current rates offered for debt with similar terms and maturities. The fair value of our long-term debt is classified as Level 2 within the fair value hierarchy. This classification is defined as a fair value determined using market-based inputs other than quoted prices that are observable for the liability, either directly or indirectly.

We have a shelf registration statement filed with the Securities and Exchange Commission that allows us to sell, in one or more future offerings, any combination of our unsecured debt securities and common stock.

In January 2015, we issued $2.5 billion of senior unsecured debt under our current shelf registration statement, comprised of $400 million of 2.30% fixed-rate notes due in February 2020, $700 million of 3.20% fixed-rate notes due in February 2025, $500 million of 3.90% fixed-rate notes due in February 2035, $650 million of 4.10% fixed-rate notes due in February 2045, and $250 million of 4.50% fixed-rate notes due in February 2065. We utilized $1.4 billion of the net proceeds to fund our acquisition of GENCO and the remaining proceeds for working capital and general corporate purposes.

A $1 billion revolving credit facility is available to finance our operations and other cash flow needs and to provide support for the issuance of commercial paper. The revolving credit agreement expires in March 2018. The agreement contains a financial covenant, which requires us to maintain a leverage ratio of adjusted debt (long-term debt, including the current portion of such debt, plus six times our last four fiscal quarters' rentals and landing fees) to capital (adjusted debt plus total common stockholders' investment) that does not exceed 70%. Our leverage ratio of adjusted debt to capital was 61% at May 31, 2015. We believe the leverage ratio covenant is our only significant restrictive covenant in our revolving credit agreement. Our revolving credit agreement contains other customary covenants that do not, individually or in the aggregate, materially restrict the conduct of our business. We are in compliance with the leverage ratio covenant and all other covenants of our revolving credit agreement and do not expect the covenants to affect our operations, including our liquidity or expected funding needs. As of May 31, 2015, no commercial paper was outstanding, and the entire $1 billion under the revolving credit facility was available for future borrowings.

We issue other financial instruments in the normal course of business to support our operations, including standby letters of credit and surety bonds. We had a total of $481 million in letters of credit outstanding at May 31, 2015, with $182 million unused under our primary $500 million letter of credit facility, and $867 million in outstanding surety bonds placed by third-party insurance providers. These instruments are required under certain U.S. self-insurance programs and are also used in the normal course of international operations. The underlying liabilities insured by these instruments are reflected in our balance sheets, where applicable. Therefore, no additional liability is reflected for the letters of credit and surety bonds themselves.