v2.4.0.8

Regulatory Matters (Narrative) (Details) (USD $) | 3 Months Ended |  |  |  |  |  | 6 Months Ended |  |  | 0 Months Ended |  |  |  | 3 Months Ended |  |  |
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Jun. 30, 2013 Columbia Of Ohio [Member] | Dec. 31, 2013 Columbia Of Ohio [Member] | Apr. 15, 2013 Columbia Of Ohio [Member] | Feb. 28, 2013 Columbia Of Ohio [Member] | May 23, 2013 Columbia Of Pennsylvania [Member] | Sep. 28, 2012 Columbia Of Pennsylvania [Member] | Jun. 30, 2013 Northern Indiana [Member] | Jul. 19, 2013 Northern Indiana [Member] | Apr. 30, 2013 Northern Indiana [Member] | Jul. 03, 2013 Columbia Of Virginia [Member] | Feb. 27, 2013 Columbia Of Maryland [Member] | Apr. 16, 2013 Columbia Of Massachusetts [Member] | Mar. 07, 2013 Columbia Of Massachusetts [Member] | Mar. 31, 2013 Columbia Transmission [Member] Customer Settlement [Member] | Jun. 03, 2013 Columbia Transmission [Member] Customer Settlement [Member] | Jan. 24, 2013 Columbia Transmission [Member] Customer Settlement [Member] | May 29, 2013 Columbia of Kentucky [Member]
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 |  |  |  |  |  |  |  |  |  |

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- Definition Amount of environmental remediation costs capitalized. These expenditures may relate to long-lived improvements in operating facilities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 25 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=28360866&loc=d3e11660-110855 + Details
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