Regulated Operations - Costs Related to Remediation of Former Operating Sites (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) site	Dec. 31, 2014 USD ($)
Environmental Exit Cost [Line Items]
Number of sites | site	46
Probabilistic model cost estimates, low estimate	$ 344
Probabilistic model cost estimates, high estimate	704
Engineering-based estimates	87
Amount recorded	431
Expected costs over next 12 months	$ 67	$ 87
Illinois [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	26
Probabilistic model cost estimates, low estimate	$ 200
Probabilistic model cost estimates, high estimate	457
Engineering-based estimates	50
Amount recorded	250
Expected costs over next 12 months	$ 32
New Jersey [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	6
Probabilistic model cost estimates, low estimate	$ 115
Probabilistic model cost estimates, high estimate	195
Engineering-based estimates	7
Amount recorded	122
Expected costs over next 12 months	$ 18
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	$ 29
Probabilistic model cost estimates, high estimate	52
Engineering-based estimates	23
Amount recorded	52
Expected costs over next 12 months	$ 12
Cost recovery period	5 years
North Carolina [Member]
Environmental Exit Cost [Line Items]
Number of sites | site	1
Probabilistic model cost estimates, low estimate	$ 0
Probabilistic model cost estimates, high estimate	0
Engineering-based estimates	7
Amount recorded	7
Expected costs over next 12 months	$ 5
Cost recovery period	0 years