Regulated Operations (Details) - Costs Related to Remediation of Former Operating Sites $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) site	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Environmental Exit Cost [Line Items]
Number of sites | site	46
Probabilistic model cost estimates, low estimate	$ 344
Probabilistic model cost estimates, high estimate	698
Engineering-based estimates	75
Amount recorded	419
Expected costs over next 12 months	$ 73	$ 87	$ 82
Illinois [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	26
Probabilistic model cost estimates, low estimate	$ 205
Probabilistic model cost estimates, high estimate	463
Engineering-based estimates	34
Amount recorded	239
Expected costs over next 12 months	$ 34
New Jersey [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	6
Probabilistic model cost estimates, low estimate	$ 105
Probabilistic model cost estimates, high estimate	177
Engineering-based estimates	9
Amount recorded	114
Expected costs over next 12 months	$ 13
Cost recovery period	7 years
Georgia And Florida [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	13
Probabilistic model cost estimates, low estimate	$ 34
Probabilistic model cost estimates, high estimate	58
Engineering-based estimates	22
Amount recorded	56
Expected costs over next 12 months	$ 18
Cost recovery period	5 years
North Carolina [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	1
Engineering-based estimates	$ 10
Amount recorded	10
Expected costs over next 12 months	$ 8