Long-Term Debt - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term debt maturities repayment of principal in next twelve months	$ 10,600,000
Long-term debt maturities repayment of principal in year two	45,600,000
Long-term debt maturities repayment of principal in year three	13,600,000
Long-term debt maturities repayment of principal in year four	25,100,000
Long-term debt maturities repayment of principal in year five	20,600,000
Long-term debt maturities repayment of principal in year five thereafter	211,700,000
Debt Instrument, Unused Borrowing Capacity, Amount	$ 250,000,000
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	$ 444,300,000
Cumulative net income with restrictions	201,500,000
Cumulative net income free of restrictions	242,800,000
Restricted net assets of consolidated subsidiaries	$ 42,200,000
Percentage of restricted net assets	8.00%
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	3.23%
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	$ 155,834,240
Cumulative net income with restrictions	(37,600,000)
Cumulative net income free of restrictions	$ 118,200,000
Uncollateralized Senior Note Due On May 2 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%
Florida Public Utilities Company [Member]
Debt Instrument [Line Items]
Cumulative net income with restrictions	$ 2,500,000