Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2015

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)	2015			2014
Asset Retirement Obligations, Beginning Balance	$	751		$	586
Liabilities Incurred	67			75
Liabilities Settled	(38		)	(101		)
Revision of Estimate	166			155
Accretion Expense	43			36
Asset Retirement Obligations, Ending Balance	$	989		$	751