Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Jan. 31, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 11		$ 112
Debt securities:
Domestic government	23		25
Corporate credit	12		10
Money market funds measured at NAV (2)	11		7
Non-qualified benefit plan trust: (2)
Money market funds	1		2
Equity securities - domestic	6		6
Debt securities—domestic government	1		1
Assets from price risk management activities: (1) (3)
Electricity	15		12
Natural gas	8		8
Total	11		7
Assets, Fair Value Disclosure	88		183
Liabilities from price risk management activities: (1) (3)
Derivative Liability		$ 5	4
Electricity	77		94
Natural gas	39		58
Total	116		156
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	11		112
Debt securities:
Domestic government	10		7
Corporate credit	0		0
Non-qualified benefit plan trust: (2)
Money market funds	1		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	29		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	13		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	14		9
Natural gas	7		8
Total	46		45
Liabilities from price risk management activities: (1) (3)
Derivative Liability			4
Electricity	8		10
Natural gas	34		51
Total	42		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	69		84
Natural gas	5		7
Total	$ 74		$ 91