Environmental Liabilities and Asset Retirement Obligations Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis
Balance at beginning of year (current and noncurrent)	$ 29		$ 25
Additions to accrual	11		5
Accretion expense	1		1
Settlements	(2)		(2)
Changes in timing and amount of estimated cash flows	(8)		0
Obligations assumed in Rockies Natural Gas Business	30	[1]	0	[1]
Balance at end of year	$ 61		$ 29

[1] ARO obligations assumed are preliminary and subject to finalization of the appraisal and full evaluation of the obligations.