Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015		Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	$ 51	$ 34	$ 111		$ 82
Amortization of:
Pension and Other Postretirement Benefit Expense				476		437
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		82	74	245	[2]	218	[2]
Interest cost		178	189	533	[2]	561	[2]
Expected return on assets		(257)	(251)	(770)	[2]	(743)	[2]
Amortization of:
Prior service cost (benefit)		3	3	10	[2]	10	[2]
Actuarial loss		142	106	427	[2]	316	[2]
Net periodic benefit cost		148	121	445	[2]	362	[2]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		30	27	89	[2]	90	[2]
Interest cost		42	42	125	[2]	144	[2]
Expected return on assets		(38)	(39)	(113)	[2]	(115)	[2]
Amortization of:
Prior service cost (benefit)		(43)	(44)	(130)	[2]	(79)	[2]
Actuarial loss		20	15	60	[2]	35	[2]
Net periodic benefit cost		11	1	31	[2]	75	[2]
CENG [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		5	5	16		11
CENG [Member] | Pension Plan, Defined Benefit [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		2	2	8		5
CENG [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		$ 3	$ 3	$ 8		$ 6

[1] Includes $4 million and $8 million, respectively, related to CENG for the three and nine months ended September 30, 2015. Includes $1 million related to CENG for the three months ended September 30, 2014 and for the period from April 1, 2014 to September 30, 2014.

[2] For the three months ended September 30, 2015, the cost for pension benefits and other postretirement benefits related to CENG were $2 million and $3 million, respectively. For the three months ended September 30, 2014, the cost for pension benefits and other postretirement benefits related to CENG were $2 million and $3 million, respectively. CENG amounts are included in the tables above. (b)For the nine months ended September 30, 2015, the cost for pension benefits and other postretirement benefits related to CENG were $8 million and $8 million, respectively. For the period of April 1, 2014 to September 30, 2014, the cost for pension benefits and other postretirement benefits related to CENG were $5 million and $6 million, respectively. CENG amounts are included in the tables above.