Summary of Significant Accounting Policies - Schedule of Capitalized and Debt Portion of AFUDC (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt portion	$ 1,129	$ 718	$ 512
Equity portion	1,943	1,154	945
AFUDC capitalized as part of utility plant	$ 3,072	$ 1,872	$ 1,457

Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.

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