Long-Term Debt - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Oct. 08, 2015	Dec. 31, 2017	Nov. 30, 2017	Mar. 31, 2017	May 31, 2016
Debt Instrument [Line Items]
Long-term debt maturities repayment of principal in next twelve months		$ 8,000
Long-term debt maturities repayment of principal in year two		10,600
Long-term debt maturities repayment of principal in year three		15,600
Long-term debt maturities repayment of principal in year four		13,600
Long-term debt maturities repayment of principal in year five		25,100
Long-term debt maturities repayment of principal in year five thereafter		132,300
Debt Instrument, Unused Borrowing Capacity, Amount		$ 80,000
Percentage of equity of total capitalization		40.00%
Fixed charge coverage ratio		1.2
Required net book value of regulated business assets, minimum percentage of consolidated total assets		50.00%
Maximum limit on payment of dividends		10.0
Cumulative consolidated net income base		$ 387,700
Cumulative net income with restrictions		178,000
Cumulative net income free of restrictions		209,700
Restricted net assets of consolidated subsidiaries		$ 43,000
Percentage of restricted net assets		9.00%
Uncollateralized Senior Note Due On Two Thousand Twenty Six [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		5.68%
First Mortgage Bond Due On Two Thousand Twenty Two [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		9.08%
Cumulative consolidated net income base		$ 142,600
Cumulative net income with restrictions		37,600
Cumulative net income free of restrictions		$ 104,900
Uncollateralized Senior Notes Due On May Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		6.43%
Uncollateralized Senior Note Two Due on December Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		3.73%
Uncollateralized Senior Notes Due On Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		3.88%
Uncollateralized Senior Note One Due On Two Thousand Seventeen [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		6.64%
Uncollateralized Senior Note Due On Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		5.50%
Uncollateralized Senior Note Due On Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage		5.93%
Notes Payable, Other Payables | Shelf Notes
Debt Instrument [Line Items]
Face amount	$ 150,000				$ 70,000
Long-term debt, interest percentage					3.25%
Term	20 years
Florida Public Utilities Company [Member]
Debt Instrument [Line Items]
Cumulative net income with restrictions		$ 2,500
MetLife [Member] | Notes Payable, Other Payables | Shelf Notes
Debt Instrument [Line Items]
Face amount				$ 150,000
New York Life [Member] | Notes Payable, Other Payables | Shelf Notes
Debt Instrument [Line Items]
Face amount				$ 100,000
Series A [Member]
Debt Instrument [Line Items]
Face amount			$ 50,000
Long-term debt, interest percentage			3.48%
Series B [Member]
Debt Instrument [Line Items]
Face amount			$ 50,000
Long-term debt, interest percentage			3.58%