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Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
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Regulatory Assets |  |
Total Regulatory Assets | $ 80,144 | $ 81,102
Regulatory Liabilities |  |
Total Regulatory Liabilities | 45,127 | 46,801
Underrecovered purchased fuel costs [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 2,219 | 911
Deferred post retirement benefits [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 17,755 | 15,640
Deferred transaction and transition costs [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 1,035 | 1,600
Deferred conversion and development costs [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 842 | 1,143
Environmental regulatory assets and expenditures [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 5,432 | 6,131
Acquisition adjustment [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 48,724 | 50,546
Loss on reacquired debt [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 1,484 | 1,576
Other [Member] |  |
Regulatory Assets |  |
Total Regulatory Assets | 2,653 | 3,555
Self insurance [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 1,212 | 1,010
Overrecovered purchased fuel costs [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 218 | 4,664
Conservation cost recovery [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 356 | 12
Rate Refund [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities |  | 1,250
Storm reserve [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 2,742 | 2,812
Accrued asset removal cost [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 38,096 | 36,584
Deferred gains [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | 1,977 |
Other [Member] |  |
Regulatory Liabilities |  |
Total Regulatory Liabilities | $ 526 | $ 469

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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