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Regulatory Matters (Narrative) (Details) (USD $)
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3 Months Ended | 6 Months Ended | 0 Months Ended | 3 Months Ended | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Jun. 30, 2013
Columbia Of Ohio [Member]
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Dec. 31, 2013
Columbia Of Ohio [Member]
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Apr. 15, 2013
Columbia Of Ohio [Member]
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Feb. 28, 2013
Columbia Of Ohio [Member]
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May 23, 2013
Columbia Of Pennsylvania [Member]
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Sep. 28, 2012
Columbia Of Pennsylvania [Member]
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Jun. 30, 2013
Northern Indiana [Member]
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Jul. 19, 2013
Northern Indiana [Member]
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Apr. 30, 2013
Northern Indiana [Member]
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Jul. 03, 2013
Columbia Of Virginia [Member]
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Feb. 27, 2013
Columbia Of Maryland [Member]
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Apr. 16, 2013
Columbia Of Massachusetts [Member]
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Mar. 07, 2013
Columbia Of Massachusetts [Member]
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Mar. 31, 2013
Columbia Transmission [Member]
Customer Settlement [Member]
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Jun. 03, 2013
Columbia Transmission [Member]
Customer Settlement [Member]
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Jan. 24, 2013
Columbia Transmission [Member]
Customer Settlement [Member]
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May 29, 2013
Columbia of Kentucky [Member]
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| Regulatory Matters [Line Items] | |||||||||||||||||
| Proposed Reduction of PIPP Rider | $ 6,700,000 | ||||||||||||||||
| Proposed Reduction to Uncollectible Expense Rider | 16,500,000 | ||||||||||||||||
| Base Chip Transition Cost | 8,200,000 | ||||||||||||||||
| Capital Expenditure Program Expenditures | 8,000,000 | ||||||||||||||||
| Proposed Revenue Increase | 29,000,000 | 77,300,000 | 5,300,000 | 30,100,000 | 16,600,000 | ||||||||||||
| Annual Revenue Increase | 55,300,000 | ||||||||||||||||
| Decoupling Revenue | 10,500,000 | ||||||||||||||||
| Annual Investment in SAVE Plan | 25,000,000 | ||||||||||||||||
| Increase to Annual Investment in SAVE Plan | 5,000,000 | ||||||||||||||||
| Five-year Period Modernization Investment | 1,500,000,000 | ||||||||||||||||
| Initial Customer Refund | 50,000,000 | ||||||||||||||||
| Customer Refund | 88,100,000 | ||||||||||||||||
| Capital Cost Recovery Mechanism Revenue Requirement | 14.00% | ||||||||||||||||
| Required Annual Capital Maintenance Expenditure | 100,000,000 | ||||||||||||||||
| Annual Capital Cost Recovery Mechanism Limit | 300,000,000 | ||||||||||||||||
| Annual Capital Cost Recovery Mechanism Limit Tolerance | 15.00% | ||||||||||||||||
| Total Capital Cost Recovery Mechanism Over Five-year Term | 1,500,000,000 | ||||||||||||||||
| Average Construction Work in Progress | 19,800,000 | ||||||||||||||||
| Transmission Assets Revenue | 1,500,000 | ||||||||||||||||
| Depreciation Rate Reduction | 1.50% | ||||||||||||||||
| Annual base rate reduction effective January 1, 2014 | 25,000,000 | ||||||||||||||||
| LNG Peaking Facility Investment | 30,000,000 | ||||||||||||||||
| Eligible Costs Recovered Under TDSIC | 80.00% | ||||||||||||||||
| Recoverable Costs Deferred for Future Recovery Under TDSIC | 20.00% | ||||||||||||||||
| Seven Year Plan of Eligible Investments Under TDSIC | 1,100,000,000 | ||||||||||||||||
| Environmental Costs Recognized, Capitalized in Period | $ 376,400,000 |