3. Debt

The following table summarizes the major components of debt at each balance sheet date (in millions) and provides the maturities and interest rate ranges of each major category as of September 30, 2012:


	September 30, 2012		December 31, 2011
Revolving credit facility (weighted average interest rate of 1.5% at December 31, 2011)	$	—	$	150
Letter of credit facilities		—		—
Canadian credit facility (weighted average effective interest rate of 2.0% at September 30, 2012 and 1.8% at December 31, 2011)		117		137
Senior notes and debentures, maturing through 2039, interest rates ranging from 2.6% to 7.75% (weighted average interest rate of 5.8% at September 30, 2012 and 6.0% at December 31, 2011)		6,711		6,228
Tax-exempt bonds maturing through 2041, fixed and variable interest rates ranging from 0.2% to 7.4% (weighted average interest rate of 2.9% at September 30, 2012 and 3.1% at December 31, 2011)		2,680		2,771
Tax-exempt project bonds, maturing through 2029, fixed and variable interest rates ranging from 0.2% to 3.4% (weighted average interest rate of 1.4% at September 30, 2012 and 1.3% at December 31, 2011)		86		86
Capital leases and other, maturing through 2055, interest rates up to 12%		398		384

		9,992		9,756
Current portion of long-term debt		826		631

	$	9,166	$	9,125

Debt Classification

As of September 30, 2012, we had $708 million of debt maturing within the next twelve months, including U.S. $117 million of advances outstanding under our Canadian credit facility, $400 million of 6.375% senior notes that mature in November 2012 and $141 million of tax-exempt bonds. We also had $663 million of tax-exempt borrowings subject to repricing within the next twelve months. We have classified $545 million of these tax-exempt borrowings as long-term based on our intent and ability to fund any failed remarketings with available capacity under our revolving credit facility.

Revolving Credit and Letter of Credit Facilities

As of September 30, 2012, we had an aggregate committed capacity of $2.5 billion for letters of credit under various credit facilities. Our $2.0 billion revolving credit facility expires in May 2016 and is our primary source of letter of credit capacity. Our remaining letter of credit capacity is provided under facilities with terms that extend from June 2013 to June 2015. As of September 30, 2012, we had an aggregate of $1.5 billion of letters of credit outstanding under various credit facilities. Approximately $1.0 billion of these letters of credit have been issued under our revolving credit facility.

Debt Borrowings and Repayments

Revolving credit facility — During the nine months ended September 30, 2012, we have had net repayments under our revolving credit facility of $150 million.

Canadian credit facility — We repaid $27 million of net advances under our Canadian credit facility during the nine months ended September 30, 2012.

Senior notes — In September 2012, we issued $500 million of 2.9% senior notes due September 15, 2022. The net proceeds from the debt issuance were $495 million. We intend to use a portion of these proceeds to repay $400 million of 6.375% senior notes that mature in November 2012. All remaining proceeds will be used for general corporate purposes. Pending repayment of the senior notes due November 2012, we used a portion of the proceeds to repay borrowings outstanding under our revolving credit facility.

Tax-exempt bonds — During the nine months ended September 30, 2012, we repaid $91 million of our tax-exempt bonds with available cash at their scheduled maturities. In addition, we issued $43 million of tax-exempt bonds, the proceeds of which were used to repay tax-exempt bonds at their scheduled maturities.