Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010 Y	Dec. 31, 2009
Current
Current Federal	$ (56.0)	$ (46.9)	$ 891.5
Current State	23.2	102.0	260.4
Current taxes charged to expense	(32.8)	55.1	1,151.9
Deferred
Deferred Federal	(134.1)	(521.4)	1,474.5
Deferred State	(59.7)	(194.9)	372.5
Deferred taxes (credited) charged to expense	(193.7)	(716.3)	1,847.0
Investment tax credit adjustments	(4.3)	(4.5)	(12.1)
Total income taxes	(230.9)	(665.7)	2,986.8
Reconciliation of Income Taxes Computed at Statutory Federal Rate to Total Income Taxes
(Loss) Income from Continuing Operations Before Income Taxes	(537.7)	(1,597.5)	7,490.2
Statutory federal income tax rate (as a percent)	35.00%	35.00%	35.00%
Income taxes computed at statutory federal rate	(188.2)	(559.1)	2,621.6
Increases (decreases) in income taxes due to
State income taxes, net of federal income tax benefit	(23.8)	(60.4)	411.0
Merger-related transaction costs			(79.3)
Interest expense on mandatorily redeemable preferred stock			23.7
Qualified decommissioning impairment losses			3.1
Amortization of deferred investment tax credits	(4.3)	(4.5)	(12.1)
Noncontrolling interest operating results	(7.1)	(13.1)	(16.4)
Nondeductible international losses			19.2
Other	(7.5)	(28.6)	16.0
Total income taxes	(230.9)	(665.7)	2,986.8
Effective income tax rate (as a percent)	42.90%	41.70%	39.90%
Electric regulatory asset, amortization period (in years)		5
Unrecognized tax benefits
Total unrecognized tax benefits, balance at beginning of period	239.8	312.5
Increases in tax positions related to the current year	3.1	5.9
Increases in tax positions related to prior years	29.6	26.0
Reductions in tax positions related to prior years	(90.6)	(104.0)
Reductions in tax positions as a result of a lapse of the applicable statute of limitations	(0.8)	(0.6)
Total unrecognized tax benefits, balance at end of period	181.1	239.8	312.5
Amount of potential decrease in income tax expense if unrecognized tax benefits were realized	169
Amount of decrease in income tax expense if unrecognized tax benefits from disallowed state tax refund claims subject to appeals were realized	169
State tax refund claims	55.9
Expected decrease in unrecognized tax benefits within the next year	108
Interest and penalties recorded as tax (benefit) expense	6.1	(6.3)	12.8
Accrued interest and penalties recognized in balance sheet	22.9	16.8
Baltimore Gas and Electric Company
Current
Current taxes charged to expense	(71.3)	(202.0)	(119.8)
Deferred
Deferred taxes (credited) charged to expense	145.8	300.2	184.7
Total income taxes	73.5	97.1	63.8
Reconciliation of Income Taxes Computed at Statutory Federal Rate to Total Income Taxes
(Loss) Income from Continuing Operations Before Income Taxes	209.2	244.7	154.5
Statutory federal income tax rate (as a percent)	35.10%	39.70%	41.30%
Increases (decreases) in income taxes due to
Total income taxes	73.5	97.1	63.8
Effective income tax rate (as a percent)	35.00%
Deferred tax liabilities
Net property, plant and equipment	1,220.1	1,152.3
Regulatory assets, net	282.6	256.8
Other	21.6	(80.0)
Total deferred tax liabilities	1,524.3	1,329.1
Deferred tax assets
Defined benefit obligations	(93.1)	(79.7)
Deferred investment tax credits	3.1	3.2
Other	71.1	20.6
Total deferred tax assets	(18.9)	(55.9)
Total deferred tax liability, net	1,543.2	1,385.0
Less: Current portion of deferred tax (asset)/liability	59.0	30.1
Long-term portion of deferred tax liability, net	1,484.2	1,354.9
Unrecognized tax benefits
Total unrecognized tax benefits, balance at beginning of period	72.9
Total unrecognized tax benefits, balance at end of period	11.4	72.9
Accrued interest and penalties recognized in balance sheet	1.2	3.8
Constellation Energy.
Deferred tax liabilities
Net property, plant and equipment	2,020.4	1,768.3
Regulatory assets, net	282.6	256.8
Derivative assets and liabilities, net	(84.6)	(34.1)
Investment in CENG	604.5	1,044.3
Other	156.3	12.1
Total deferred tax liabilities	2,979.2	3,047.4
Deferred tax assets
Defined benefit obligations	305.0	249.0
Financial investments and hedging instruments	217.6	111.4
Asset retirement obligation	10.9	10.9
Deferred investment tax credits	9.6	10.9
Other	263.0	118.9
Total deferred tax assets	806.1	501.1
Total deferred tax liability, net	2,173.1	2,546.3
Less: Current portion of deferred tax (asset)/liability	(132.0)	56.5
Long-term portion of deferred tax liability, net	$ 2,305.1	$ 2,489.8