Fair Value Measurements (Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Assets:
Nuclear decommissioning trust		$ 874	$ 893
Nuclear decommissioning trust – equity securities [Member]
Assets:
Nuclear decommissioning trust	[1]	560	592
Nuclear decommissioning trust – debt securities [Member]
Assets:
Nuclear decommissioning trust	[2]	314	301
Commodity contracts [Member]
Assets:
Gross commodity contracts		423	497
Liabilities:
Gross commodity contracts		164	317
Fair Value, Measurements, Recurring [Member]
Assets:
Commodity contracts reclassification	[3]	16
Liabilities:
Commodity contracts reclassification	[3]	16
Fair Value, Measurements, Recurring [Member] | Commodity contracts [Member]
Assets:
Commodity contracts reclassification	[3]	16
Liabilities:
Commodity contracts reclassification	[3]	16
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Total assets		683	777
Liabilities:
Total liabilities		108	278
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Nuclear decommissioning trust – equity securities [Member]
Assets:
Nuclear decommissioning trust	[4]	355	375
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Nuclear decommissioning trust – debt securities [Member]
Assets:
Nuclear decommissioning trust	[4]		0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodity contracts [Member]
Assets:
Gross commodity contracts		328	402
Liabilities:
Gross commodity contracts		108	278
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Total assets		564	564
Liabilities:
Total liabilities		33	25
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Nuclear decommissioning trust – equity securities [Member]
Assets:
Nuclear decommissioning trust	[4]	205	217
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Nuclear decommissioning trust – debt securities [Member]
Assets:
Nuclear decommissioning trust	[4]	314	301
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodity contracts [Member]
Assets:
Gross commodity contracts		45	46
Liabilities:
Gross commodity contracts		33	25
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Total assets	[5]	34	49
Liabilities:
Total liabilities	[5]	7	14
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Nuclear decommissioning trust – equity securities [Member]
Assets:
Nuclear decommissioning trust	[4],[5]	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Nuclear decommissioning trust – debt securities [Member]
Assets:
Nuclear decommissioning trust	[4],[5]	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity contracts [Member]
Assets:
Gross commodity contracts	[5]	34	49
Liabilities:
Gross commodity contracts	[5]	7	14
Total [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Total assets		1,297	1,390
Liabilities:
Total liabilities		164	317
Total [Member] | Fair Value, Measurements, Recurring [Member] | Nuclear decommissioning trust – equity securities [Member]
Assets:
Nuclear decommissioning trust	[4]	560	592
Total [Member] | Fair Value, Measurements, Recurring [Member] | Nuclear decommissioning trust – debt securities [Member]
Assets:
Nuclear decommissioning trust	[4]	314	301
Total [Member] | Fair Value, Measurements, Recurring [Member] | Commodity contracts [Member]
Assets:
Commodity contracts		423	497
Liabilities:
Commodity contracts		$ 164	$ 317

[1] The investment objective for equity securities is to invest tax efficiently and to match the performance of the S&P 500 Index.

[2] The investment objective for debt securities is to invest in a diversified tax efficient portfolio with an overall portfolio rating of AA or above as graded by S&P or Aa2 by Moody's Investors Services, Inc. The debt securities are heavily weighted with municipal bonds. The debt securities had an average coupon rate of 3.58% and 4.35% at September 30, 2015 and December 31, 2014, respectively, and an average maturity of 7 years and 6 years at September 30, 2015 and December 31, 2014, respectively.

[3] Fair values are determined on a contract basis, but certain contracts result in a current asset and a noncurrent liability, or vice versa, as presented in the condensed consolidated balance sheets.

[4] The nuclear decommissioning trust investment is included in the investments line in the condensed consolidated balance sheets. See Note 15.

[5] See table below for description of Level 3 assets and liabilities.