Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets (Liabilities) related to Retirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial loss (gain)		$ 476	$ 1,639	$ (1,169)
Amortization of actuarial loss		(571)	(420)	(562)
Current year prior service (credit) cost		0	0	0
Amortization of prior service (cost) credit		(13)	(14)	(14)
Settlements		(2)	(2)	(8)
Total recognized in AOCI and regulatory assets (liabilities)	[1]	(110)	1,203	(1,753)
Defined benefit plan, amounts recognized in OCI, before tax		64	788	1,071
Defined benefit plan, amounts recognized in regulatory assets, before tax		46	415	682
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial loss (gain)		(194)	561	(628)
Amortization of actuarial loss		(80)	(50)	(83)
Current year prior service (credit) cost		(23)	(1,012)	15
Amortization of prior service (cost) credit		174	122	19
Settlements		0	0	0
Total recognized in AOCI and regulatory assets (liabilities)	[1]	(123)	(379)	(677)
Defined benefit plan, amounts recognized in OCI, before tax		63	162	352
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ 60	$ 217	$ 325

[1] )Of the $110 million gain related to pension benefits, $64 million and $46 million were recognized in AOCI and regulatory assets, respectively, during 2015. Of the $123 million gain related to other postretirement benefits, $63 million and $60 million were recognized in AOCI and regulatory assets (liabilities), respectively, during 2015. 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, respectively, during 2013.