Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended				9 Months Ended				3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014 Pension Plan, Defined Benefit [Member]		Sep. 30, 2013 Pension Plan, Defined Benefit [Member]		Sep. 30, 2014 Pension Plan, Defined Benefit [Member]		Sep. 30, 2013 Pension Plan, Defined Benefit [Member]		Sep. 30, 2014 Other Postretirement Benefit Plan, Defined Benefit [Member]		Sep. 30, 2013 Other Postretirement Benefit Plan, Defined Benefit [Member]		Sep. 30, 2014 Other Postretirement Benefit Plan, Defined Benefit [Member]		Sep. 30, 2013 Other Postretirement Benefit Plan, Defined Benefit [Member]		Sep. 30, 2014 CENG [Member] Pension Plan, Defined Benefit [Member]	Sep. 30, 2014 CENG [Member] Pension Plan, Defined Benefit [Member]	Sep. 30, 2014 CENG [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 CENG [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 CENG [Member]	Sep. 30, 2014 CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	$ 34	[1]	$ 18	[1]	$ 82	[1]	$ 61	[1]																					$ 1	$ 1
Service cost									74	[2]	79	[2]	218	[3]	238	[3]	27	[2]	41	[2]	90	[3]	122	[3]
Interest cost									189	[2]	163	[2]	561	[3]	488	[3]	42	[2]	48	[2]	144	[3]	145	[3]
Expected return on assets									(251)	[2]	(253)	[2]	(743)	[3]	(761)	[3]	(39)	[2]	(33)	[2]	(115)	[3]	(99)	[3]
Amortization of:
Prior service cost (benefit)									3	[2]	3	[2]	10	[3]	10	[3]	(44)	[2]	(4)	[2]	(79)	[3]	(14)	[3]
Actuarial loss									106	[2]	140	[2]	316	[3]	421	[3]	15	[2]	20	[2]	35	[3]	62	[3]
Settlement charges									0	[2]	9	[2]	0	[3]	9	[3]	0	[2]	0	[2]	0	[3]	0	[3]
Net periodic benefit cost									121	[2]	141	[2]	362	[3]	405	[3]	1	[2]	72	[2]	75	[3]	216	[3]
Pension and Other Postretirement Benefit Costs																									$ 2	$ 5	$ 3	$ 6

[1] 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. CENG is not included in the 2013 amounts.

[2] 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 is not included in the 2013 amounts.

[3] For the period 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 is not included in the 2013 amounts.