Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets (Liabilities) related to Retirement Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	$ 1,639		$ (1,169)		$ 1,693
Amortization of actuarial loss	(420)		(562)		(450)
Current year prior service (credit) cost	0		0		1
Amortization of prior service (cost) credit	(14)		(14)		(15)
Current year transition (asset) obligation	0		0		0
Amortization of transition asset (obligation)	0		0		0
Curtailments	0		0		(10)
Settlements	(2)		(8)		(31)
Total recognized in AOCI and regulatory assets (liabilities)	1,203	[1]	(1,753)	[1]	1,188	[1]
Defined benefit plan, amounts recognized in OCI, before tax	788		1,071		283
Defined benefit plan, amounts recognized in regulatory assets, before tax	415		682		904
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	561		(628)		304
Amortization of actuarial loss	(50)		(83)		(81)
Current year prior service (credit) cost	(1,012)		15		(109)
Amortization of prior service (cost) credit	122		19		17
Current year transition (asset) obligation	0		0		1
Amortization of transition asset (obligation)	0		0		(11)
Curtailments	0		0		(1)
Settlements	0		0		0
Total recognized in AOCI and regulatory assets (liabilities)	(379)	[1]	(677)	[1]	120	[1]
Defined benefit plan, amounts recognized in OCI, before tax	162		352		39
Defined benefit plan, amounts recognized in regulatory assets, before tax	$ 217		$ 325		$ 81

[1] )Of the $1,203 million loss related to pension benefits, $788 million and $415 million were recognized in AOCI and regulatory assets, respectively, during 2014. Of the $379 million gain related to other postretirement benefits, $162 million and $217 million were recognized in AOCI and regulatory assets (liabilities), respectively, during 2014. Of the $1,753 million gain related to pension benefits, $1,071 million and $682 million were recognized in AOCI and regulatory assets, respectively, during 2013. Of the $677 million gain related to other postretirement benefits, $352 million and $325 million were recognized in AOCI and regulatory assets (liabilities), respectively, during 2013. Of the $1,188 million loss related to pension benefits, $283 million and $904 million were recognized in AOCI and regulatory assets, respectively, during 2012. Of the $120 million loss related to other postretirement benefits, $39 million and $81 million were recognized in AOCI and regulatory assets, respectively, during 2012.