Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2014

Asset Retirement Obligation Disclosure [Abstract]

Schedule of Change in Asset Retirement Obligation

Changes in asset retirement obligations were as follows:

	Year Ended December 31,
(millions)	2014			2013
Asset Retirement Obligations, Beginning Balance	$	586		$	402
Liabilities Incurred	75			90
Liabilities Settled	(101		)	(41		)
Revision of Estimate	155			156
Accretion Expense	36			28
Other	—			(49		)
Asset Retirement Obligations, Ending Balance	$	751		$	586