Summary of Significant Accounting Policies - Schedule of Regulatory Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
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