Long-Term Debt - Additional Information (Detail)		12 Months Ended
	Oct. 08, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 05, 2013 USD ($)	Jun. 30, 2011	Jun. 30, 2010 USD ($)
Debt Instrument [Line Items]
Long-term debt maturities repayment of principal in next twelve months		$ 9,151,000
Long-term debt maturities repayment of principal in year two		12,099,000
Long-term debt maturities repayment of principal in year three		9,421,000
Long-term debt maturities repayment of principal in year four		11,245,000
Long-term debt maturities repayment of principal in year five		15,600,000
Long-term debt maturities repayment of principal in year five thereafter		101,000,000
Long-term Debt		$ 153,700,000	$ 161,500,000
Debt Instrument, Number of Series					2
Percentage of equity of total capitalization		40.00%
Fixed charge coverage ratio		1.2
Debt Instrument, Restrictive Covenants, Required Net Book Value of Regulated Business Assets Percentage of Consolidated Total Assets		50.00%
Maximum limit on payment of dividends		10.0
Cumulative consolidated net income base		$ 285,000,000
Cumulative net income with restrictions		138,400,000
Cumulative net income free of restrictions		146,600,000
Restricted net assets of consolidated subsidiaries		$ 47,100,000
Percentage of restricted net assets		13.00%
Long Term Debt Commitment						$ 36,000,000
Uncollateralized Senior Note Due On Two Thousand Twenty Six [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 29,000,000	29,000,000
First Mortgage Bond Due On Two Thousand Twenty Two [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 7,973,000	7,969,000
Maximum limit on payment of dividends		2500000
Cumulative consolidated net income base		$ 116,300,000
Cumulative net income with restrictions		37,600,000
Cumulative net income free of restrictions		78,700,000
Aggregate Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Long Term Debt Commitment				$ 70,000,000
Uncollateralized Senior Notes Due On May Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term Debt		7,000,000	7,000,000
Uncollateralized Senior Note Two Due on December Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term Debt		20,000,000	20,000,000
Uncollateralized Senior Notes Due On Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Long-term Debt		50,000,000	50,000,000
Uncollateralized Senior Note One Due On Two Thousand Seventeen [Member]
Debt Instrument [Line Items]
Long-term Debt		5,456,000	8,182,000
Uncollateralized Senior Note Due On Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term Debt		10,000,000	12,000,000
Uncollateralized Senior Note Due On Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 24,000,000	$ 27,000,000
Notes Payable, Other Payables [Member] | Shelf Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 150,000,000
Debt Instrument, Term	20 years
Period To Purchase Unsecured Senior Notes	3 years