Long-Term Debt - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	May 31, 2013	Jun. 30, 2011 series	Jun. 30, 2010	Sep. 30, 2013 5.68% note, due June 30, 2026 [Member]	Dec. 31, 2012 5.68% note, due June 30, 2026 [Member]	Jun. 30, 2011 5.68% note, due June 30, 2026 [Member]	Sep. 05, 2013 Uncollateralized Senior Notes Due On Two Thousand Twenty Nine [Member]	Sep. 30, 2013 Unsecured senior notes 6.43% [Member]	May 02, 2013 Unsecured senior notes 6.43% [Member]	Dec. 31, 2012 Unsecured senior notes 6.43% [Member]	May 31, 2013 First mortgage bond one [Member]	May 31, 2013 First mortgage bond two [Member]	Sep. 05, 2013 Aggregate Unsecured Senior Notes [Member]	Sep. 05, 2013 Uncollateralized Senior Notes Due On Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long term debt maximum borrowing capacity						$ 36,000,000				$ 50,000,000						$ 70,000,000	$ 20,000,000
Total long-term debt			108,500,000				29,000,000	29,000,000	29,000,000		7,000,000	7,000,000	0
Long-term debt, interest percentage							5.68%		5.68%	3.88%	6.43%	6.40%		9.57%	10.03%		3.73%
Number of series					2
Amount paid in excess of carrying value deferred as regulatory asset				93,000
Repayments of Long-term Debt	8,609,000	1,459,000
Scheduled Repayment of Long Term Debt										$ 5,000,000							$ 2,000,000