Compensation and Benefit Plans (Plan Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended							12 Months Ended
	Dec. 31, 2012 Master Trust Investments Level 3	Dec. 31, 2011 Master Trust Investments Level 3	Dec. 31, 2012 Pension Plans	Dec. 31, 2010 Pension Plans	Dec. 31, 2012 Pension Plans Midwest Generation LLC	Dec. 31, 2011 Pension Plans Midwest Generation LLC	Dec. 31, 2010 Pension Plans Midwest Generation LLC	Dec. 31, 2012 Pension Plans U.S. Equities	Dec. 31, 2012 Pension Plans Non-U.S. Equities	Dec. 31, 2012 Pension Plans Fixed Income	Dec. 31, 2012 Pension Plans Opportunistic and/or Alternative Investments	Dec. 31, 2012 Pension Plans Other Investments	Dec. 31, 2012 Pension Plans Master Trust Investments	Dec. 31, 2011 Pension Plans Master Trust Investments	Dec. 31, 2012 Pension Plans Master Trust Investments Level 1	Dec. 31, 2011 Pension Plans Master Trust Investments Level 1	Dec. 31, 2012 Pension Plans Master Trust Investments Level 2	Dec. 31, 2011 Pension Plans Master Trust Investments Level 2	Dec. 31, 2012 Pension Plans Master Trust Investments Level 3	Dec. 31, 2011 Pension Plans Master Trust Investments Level 3	Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate stocks		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks Russell Index	Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks MSCI Index	Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate stocks Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate stocks Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate stocks Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate stocks Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Partnerships/joint ventures		Dec. 31, 2011 Pension Plans Master Trust Investments Partnerships/joint ventures		Dec. 31, 2012 Pension Plans Master Trust Investments Partnerships/joint ventures Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Partnerships/joint ventures Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Partnerships/joint ventures Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Partnerships/joint ventures Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Partnerships/joint ventures Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Partnerships/joint ventures Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Asset backed securities including distressed mortgages Level 3	Dec. 31, 2011 Pension Plans Master Trust Investments Asset backed securities including distressed mortgages Level 3	Dec. 31, 2012 Pension Plans Master Trust Investments Common/collective funds		Dec. 31, 2011 Pension Plans Master Trust Investments Common/collective funds		Dec. 31, 2012 Pension Plans Master Trust Investments Common/collective funds Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Common/collective funds Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Common/collective funds Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Common/collective funds Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Common/collective funds Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Common/collective funds Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Equity index fund S&P 500 Index	Dec. 31, 2011 Pension Plans Master Trust Investments Equity index fund S&P 500 Index	Dec. 31, 2012 Pension Plans Master Trust Investments Equity index fund EAFE Index	Dec. 31, 2011 Pension Plans Master Trust Investments Equity index fund EAFE Index	Dec. 31, 2012 Pension Plans Master Trust Investments Undervalued securities Russell Index	Dec. 31, 2011 Pension Plans Master Trust Investments Undervalued securities Russell Index	Dec. 31, 2012 Pension Plans Master Trust Investments Non-index fund	Dec. 31, 2011 Pension Plans Master Trust Investments Non-index fund	Dec. 31, 2012 Pension Plans Master Trust Investments Global asset allocation fund	Dec. 31, 2011 Pension Plans Master Trust Investments Global asset allocation fund	Dec. 31, 2012 Pension Plans Master Trust Investments Corporate bonds		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate bonds		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate bonds Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate bonds Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate bonds Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate bonds Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Corporate bonds Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Corporate bonds Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Collateralized mortgage obligations and other asset backed securities	Dec. 31, 2011 Pension Plans Master Trust Investments Collateralized mortgage obligations and other asset backed securities	Dec. 31, 2012 Pension Plans Master Trust Investments U.S. government and agency securities		Dec. 31, 2011 Pension Plans Master Trust Investments U.S. government and agency securities		Dec. 31, 2012 Pension Plans Master Trust Investments U.S. government and agency securities Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments U.S. government and agency securities Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments U.S. government and agency securities Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments U.S. government and agency securities Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments U.S. government and agency securities Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments U.S. government and agency securities Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Other investment entities		Dec. 31, 2011 Pension Plans Master Trust Investments Other investment entities		Dec. 31, 2012 Pension Plans Master Trust Investments Other investment entities Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Other investment entities Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Other investment entities Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Other investment entities Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Other investment entities Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Other investment entities Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Registered investment companies		Dec. 31, 2011 Pension Plans Master Trust Investments Registered investment companies		Dec. 31, 2012 Pension Plans Master Trust Investments Registered investment companies Level 1		Dec. 31, 2011 Pension Plans Master Trust Investments Registered investment companies Level 1		Dec. 31, 2012 Pension Plans Master Trust Investments Registered investment companies Level 2		Dec. 31, 2011 Pension Plans Master Trust Investments Registered investment companies Level 2		Dec. 31, 2012 Pension Plans Master Trust Investments Registered investment companies Level 3		Dec. 31, 2011 Pension Plans Master Trust Investments Registered investment companies Level 3		Dec. 31, 2012 Pension Plans Master Trust Investments Interest-bearing cash	Dec. 31, 2011 Pension Plans Master Trust Investments Interest-bearing cash	Dec. 31, 2012 Pension Plans Master Trust Investments Interest-bearing cash Level 1	Dec. 31, 2011 Pension Plans Master Trust Investments Interest-bearing cash Level 1	Dec. 31, 2012 Pension Plans Master Trust Investments Interest-bearing cash Level 2	Dec. 31, 2011 Pension Plans Master Trust Investments Interest-bearing cash Level 2	Dec. 31, 2012 Pension Plans Master Trust Investments Interest-bearing cash Level 3	Dec. 31, 2011 Pension Plans Master Trust Investments Interest-bearing cash Level 3	Dec. 31, 2012 Pension Plans Master Trust Investments Other Investments	Dec. 31, 2011 Pension Plans Master Trust Investments Other Investments	Dec. 31, 2012 Pension Plans Master Trust Investments Other Investments Level 1	Dec. 31, 2011 Pension Plans Master Trust Investments Other Investments Level 1	Dec. 31, 2012 Pension Plans Master Trust Investments Other Investments Level 2	Dec. 31, 2011 Pension Plans Master Trust Investments Other Investments Level 2	Dec. 31, 2012 Pension Plans Master Trust Investments Other Investments Level 3	Dec. 31, 2011 Pension Plans Master Trust Investments Other Investments Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocations for plan assets (as a percent)								30.00%	16.00%	35.00%	15.00%	4.00%
Permissible range of asset class portfolio weights, plus or minus (as a percent)			3.00%
Percentage of premium based on assumed average equity risk premium over cash (as a percent)			5.00%
Percentage of premium included in estimated rate of return of public equity (as a percent)			2.00%
Fair value of plan assets, before receivables (payables)													$ 3,580	$ 3,192	$ 1,108	$ 829	$ 2,058	$ 1,915	$ 414	$ 448	$ 743	[1]	$ 642	[1]			$ 743	[1]	$ 642	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 580	[2]	$ 588	[2]	$ 0	[2]	$ 0	[2]	$ 166	[2]	$ 140	[2]	$ 414	[2]	$ 448	[2]			$ 635	[3]	$ 582	[3]	$ 0	[3]	$ 0	[3]	$ 635	[3]	$ 582	[3]	$ 0	[3]	$ 0	[3]											$ 508	[4]	$ 497	[4]	$ 0	[4]	$ 0	[4]	$ 508	[4]	$ 497	[4]	$ 0	[4]	$ 0	[4]	$ 65	$ 53	$ 592	[5]	$ 455	[5]	$ 242	[5]	$ 104	[5]	$ 350	[5]	$ 351	[5]	$ 0	[5]	$ 0	[5]	$ 271	[6]	$ 247	[6]	$ 0	[6]	$ 0	[6]	$ 271	[6]	$ 247	[6]	$ 0	[6]	$ 0	[6]	$ 126	[7]	$ 108	[7]	$ 98	[7]	$ 79	[7]	$ 28	[7]	$ 29	[7]	$ 0	[7]	$ 0	[7]	$ 24	$ 5	$ 24	$ 5	$ 0	$ 0	$ 0	$ 0	$ 101	$ 68	$ 1	$ (1)	$ 100	$ 69	$ 0	$ 0
Receivables and payables, net													(38)	(39)
Net plan assets available for benefits	414	448	215	164	149	121	109						3,542	3,153
Percentage of performance to benchmark (as a percent)																									60.00%	40.00%																																															29.00%	29.00%	11.00%	10.00%	28.00%	27.00%	25.00%	23.00%
Actual plan asset allocation (as a percent)																																																							56.00%	55.00%																									6.00%	8.00%
Fair value of assets below investment grade																																																																																																			7	10
Percentage of publicly traded equity investments located in United States (as a percent)													66.00%	69.00%
Change in fair value of Level 3 investments
Fair value of plan assets at beginning of period	448	345	177	164	149	121	109						3,153
Actual return on plan assets:
Relating to assets still held at end of period	88	6
Relating to assets sold during the period	13	22
Purchases	98	130
Dispositions	(233)	(55)
Transfers in and /or out of Level 3	0	0
Fair value of plan assets at end of period	$ 414	$ 448	$ 215	$ 164	$ 149	$ 121	$ 109						$ 3,542	$ 3,153

[1] Corporate stocks are diversified. For both 2012 and 2011, performance is primarily benchmarked against the Russell Indexes (60%) and Morgan Stanley Capital International (MSCI) index (40%).

[2] At December 31, 2012 and 2011, respectively, the common/collective assets were invested in equity index funds that seek to track performance of the Standard and Poor's (S&P 500) Index (29% and 29%), Russell 200 and Russell 1000 indexes (28% and 27%) and the MSCI Europe, Australasia and Far East (EAFE) Index (11% and 10%). A non-index U.S. equity fund representing 25% and 23% of this category for 2012 and 2011, respectively, is actively managed. Another fund representing 6% and 8% of this category for 2012 and 2011, respectively, is a global asset allocation fund.

[3] Level 1 U.S. government and agency securities are U.S. treasury bonds and notes. Level 2 primarily relates to the Federal National Mortgage Association and the Federal Home Loan Mortgage Corporation.

[4] Partnerships/joint venture Level 2 investments consist primarily of a partnership which invests in publicly traded fixed income securities, primarily from the banking and finance industry and U.S. government agencies. At December 31, 2012 and 2011, respectively, approximately 56% and 55% of the Level 3 partnerships are invested in (1) asset backed securities, including distressed mortgages and (2) commercial and residential loans and debt and equity of banks. The remaining Level 3 partnerships are invested in small private equity and venture capital funds. Investment strategies for these funds include branded consumer products, early stage technology, California geographic focus, and diversified US and non-US fund-of-funds.

[5] Corporate bonds are diversified. At December 31, 2012 and 2011, respectively, this category includes $65 million and $53 million for collateralized mortgage obligations and other asset backed securities of which $7 million and $10 million are below investment grade.

[6] Other investment entities were primarily invested in (1) emerging market equity securities, (2) a hedge fund that invests through liquid instruments in a global diversified portfolio of equity, fixed income, interest rate, foreign currency and commodities markets, and (3) domestic mortgage backed securities.

[7] Level 1 of registered investment companies consisted of a global equity mutual fund which seeks to outperform the MSCI World Total Return Index. Level 2 primarily consisted of short-term, emerging market, high yield bond funds and government inflation-indexed bonds and short-term bond fund.