Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 31, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 82		$ 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
Eqauity securities - domestic	6		6
Debt securities—domestic government	1		1
Assets from price risk management activities: (1) (3)
Electricity	18		12
Natural gas	19		8
Total	12		7
Assets, Fair Value Disclosure	174		183
Liabilities from price risk management activities: (1) (3)
Derivative Liability		$ 5	4
Electricity	75		94
Natural gas	35		58
Total	110		156
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	82		112
Debt securities:
Domestic government	8		7
Corporate credit	0		0
Non-qualified benefit plan trust: (2)
Money market funds	2		2
Eqauity 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	99		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	14		18
Corporate credit	12		10
Non-qualified benefit plan trust: (2)
Money market funds	0		0
Eqauity securities - domestic	0		0
Debt securities—domestic government	0		0
Assets from price risk management activities: (1) (3)
Electricity	14		9
Natural gas	19		8
Total	59		45
Liabilities from price risk management activities: (1) (3)
Derivative Liability			4
Electricity	6		10
Natural gas	30		51
Total	36		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
Eqauity securities - domestic	0		0
Debt securities—domestic government	0		0
Assets from price risk management activities: (1) (3)
Electricity	4		3
Natural gas	0		0
Total	4		3
Liabilities from price risk management activities: (1) (3)
Derivative Liability			0
Electricity	69		84
Natural gas	5		7
Total	$ 74		$ 91