Pension and Postretirement Benefits (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Baltimore Gas and Electric Company \| Net periodic pension benefit cost
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	$ 9.0	$ 8.5	$ 18.1	$ 14.8
Net periodic pension benefit cost
Components of net periodic pension and postretirement benefit cost
Service cost	15.0	9.1	26.4	18.6
Interest cost	24.6	19.7	47.2	41.4
Expected return on plan assets	(32.3)	(23.2)	(62.3)	(49.9)
Recognized net actuarial loss	11.9	8.7	24.1	16.8
Amortization of prior service cost	1.1	0.9	2.1	1.9
Amount capitalized as construction cost	(3.2)	(2.9)	(6.1)	(4.8)
Net periodic pension and postretirement benefit cost	17.1	12.3	31.4	24.0
Settlement charges excluded from net periodic pension benefit costs		1.5		1.5
Non-qualified Pension Plans
Components of net periodic pension and postretirement benefit cost
Expected benefit payments	9.4		9.4
Baltimore Gas and Electric Company \| Net periodic postretirement benefit cost
Components of net periodic pension and postretirement benefit cost
Net periodic pension and postretirement benefit cost, excluding capitalized costs	4.9	4.6	9.5	9.4
Net periodic postretirement benefit cost
Components of net periodic pension and postretirement benefit cost
Service cost	0.8	0.6	1.5	1.3
Interest cost	5.1	5.0	9.3	9.7
Amortization of transition obligation	0.5	0.6	1.0	1.1
Recognized net actuarial loss	0.2	(0.1)	0.8	0.2
Amortization of prior service cost	(0.9)	(0.7)	(1.5)	(1.4)
Amount capitalized as construction cost	(1.7)	(1.6)	(3.1)	(2.9)
Net periodic pension and postretirement benefit cost	4.0	3.8	8.0	8.0
Expected benefit payments to be made for retiree health and life insurance costs in 2011	$ 22.6		$ 22.6

The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan in the current remaining fiscal period. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions.

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- Definition

The portion of the total amount of net periodic benefit cost for defined benefit plans that was capitalized during the period.

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The total amount of net periodic benefit cost for defined benefit plans for the period, excluding capitalized costs. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments).

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- Definition

The amount of settlement charges excluded from net periodic pension benefit costs in the period.

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- Definition

The employer's best estimate of payments expected to be made for retiree health and life insurance costs in the current remaining fiscal period.

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- Definition

The net increase or decrease of changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 106
-Paragraph 518

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 87
-Paragraph 264

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 5
-Subparagraph a

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- Definition

The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 5
-Subparagraph h

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- Definition

The amount of the transition obligation or asset recognized in net periodic benefit cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 5
-Subparagraph h

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain or loss, prior service cost or credit, transition asset or obligation, and gain or loss due to settlements or curtailments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 132R
-Paragraph 5
-Subparagraph h

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

+ Details

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