Regulated Operations - Costs Related to Remediation of Former Operating Sites (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) site	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)
Environmental Exit Cost [Line Items]
Number of sites | site	46
Probabilistic model cost estimates, low estimate	$ 355
Probabilistic model cost estimates, high estimate	724
Engineering-based estimates	83
Amount recorded	438
Expected costs over next 12 months	$ 59	$ 67	$ 83
Illinois [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	26
Probabilistic model cost estimates, low estimate	$ 206
Probabilistic model cost estimates, high estimate	470
Engineering-based estimates	46
Amount recorded	252
Expected costs over next 12 months	$ 27
New Jersey [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	6
Probabilistic model cost estimates, low estimate	$ 111
Probabilistic model cost estimates, high estimate	190
Engineering-based estimates	8
Amount recorded	119
Expected costs over next 12 months	$ 14
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	$ 38
Probabilistic model cost estimates, high estimate	64
Engineering-based estimates	24
Amount recorded	62
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	$ 5
Amount recorded	5
Expected costs over next 12 months	$ 0