The following table summarizes the major components of debt as of each balance sheet date (in millions) and provides the maturities and interest rate ranges of each major category as of March 31, 2018:


	March 31,		December 31,
	2018		2017
$2.25 billion revolving credit facility, maturing July 2020	$	28	$	—
Commercial paper program (weighted average interest rate of 2.3% as of March 31, 2018 and 1.9% as of December 31, 2017)		989		515
Canadian term loan and revolving credit facility, maturing March 2019 (weighted average effective interest rate of 2.8% as of March 31, 2018 and 2.5% as of December 31, 2017)		99		113
Senior notes, maturing through 2045, interest rates ranging from 2.4% to 7.75% (weighted average interest rate of 4.3% as of March 31, 2018 and December 31, 2017)		6,186		6,184
Tax-exempt bonds, maturing through 2045, fixed and variable interest rates ranging from 1.35% to 5.7% (weighted average interest rate of 2.1% as of March 31, 2018 and 2.0% as of December 31, 2017)		2,332		2,352
Capital leases and other, maturing through 2040, interest rates up to 12%		323		327
		9,957		9,491
Current portion of long-term debt		1,056		739
	$	8,901	$	8,752