Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jan. 31, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 0		$ 112
Debt securities:
Domestic government	22		25
Corporate credit	12		10
Money market funds measured at NAV (2)	12		7
Non-qualified benefit plan trust: (2)
Money market funds	2		2
Equity securities - domestic	6		6
Debt securities—domestic government	1		1
Assets from price risk management activities: (1) (3)
Electricity	7		12
Natural gas	11		8
Total	12		7
Assets, Fair Value Disclosure	73		183
Liabilities from price risk management activities: (1) (3)
Derivative Liability	0	$ 5	4
Electricity	104		94
Natural gas	22		58
Total	126		156
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	0		112
Debt securities:
Domestic government	7		7
Corporate credit	0		0
Non-qualified benefit plan trust: (2)
Money market funds	2		2
Equity securities - domestic	6		6
Debt securities—domestic government	1		1
Assets from price risk management activities: (1) (3)
Electricity	0		0
Natural gas	0		0
Total	16		128
Liabilities from price risk management activities: (1) (3)
Derivative Liability			0
Electricity	0		0
Natural gas	0		0
Total	0		0
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash equivalents	0		0
Debt securities:
Domestic government	15		18
Corporate credit	12		10
Non-qualified benefit plan trust: (2)
Money market funds	0		0
Equity securities - domestic	0		0
Debt securities—domestic government	0		0
Assets from price risk management activities: (1) (3)
Electricity	6		9
Natural gas	10		8
Total	43		45
Liabilities from price risk management activities: (1) (3)
Derivative Liability			4
Electricity	5		10
Natural gas	18		51
Total	23		65
Fair Value, Inputs, Level 3 [Member]
Assets:
Cash equivalents	0		0
Debt securities:
Domestic government	0		0
Corporate credit	0		0
Non-qualified benefit plan trust: (2)
Money market funds	0		0
Equity securities - domestic	0		0
Debt securities—domestic government	0		0
Assets from price risk management activities: (1) (3)
Electricity	1		3
Natural gas	1		0
Total	2		3
Liabilities from price risk management activities: (1) (3)
Derivative Liability			0
Electricity	99		84
Natural gas	4		7
Total	$ 103		$ 91