Retirement Benefits - Calculation of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015		Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	$ 38	$ 19	$ 60		$ 48
Amortization of:
Pension and Other Postretirement Benefit Expense				317		315
CENG [Member]
Defined Benefit Plan Disclosure [Line Items]
Savings Plan Matching Contributions	[1]	3	1	4
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		81	74	163	[2]	143	[2]
Interest cost		178	190	355	[2]	373	[2]
Expected return on assets		(256)	(251)	(513)	[2]	(492)	[2]
Amortization of:
Prior service cost (benefit)		4	4	7	[2]	7	[2]
Actuarial loss		142	104	285	[2]	209	[2]
Net periodic benefit cost		149	121
Pension Plan, Defined Benefit [Member] | Regulatory Assets [Member]
Amortization of:
Net periodic benefit cost	[2]			297		240
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		29	30	59	[2]	62	[2]
Interest cost		42	47	83	[2]	103	[2]
Expected return on assets		(37)	(38)	(75)	[2]	(76)	[2]
Amortization of:
Prior service cost (benefit)		(44)	(30)	(88)	[2]	(34)	[2]
Actuarial loss		21	12	41	[2]	20	[2]
Net periodic benefit cost		11	21
Other Postretirement Benefit Plan, Defined Benefit [Member] | Regulatory Assets [Member]
Amortization of:
Net periodic benefit cost	[2]			20		$ 75
CENG [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		6	5	11
CENG [Member] | Pension Plan, Defined Benefit [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		3	2	5
CENG [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Amortization of:
Pension and Other Postretirement Benefit Costs		$ 3	$ 3	$ 6

[1] Includes $3 million and $4 million, respectively, related to CENG for the three and six months ended June 30, 2015. For the period of April 1, 2014 to June 30, 2014, amounts include $1 million related to CENG.

[2] For the three months ended June 30, 2015, the cost for pension benefits and other postretirement benefits related to CENG were $3 million and $3 million, respectively. For the six months ended June 30, 2015, the cost for pension benefits and other postretirement benefits related to CENG were $5 million and $6 million, respectively. For the period of April 1, 2014 to June 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.