Long-Term Debt - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt maturities repayment of principal in next twelve months	$ 45,600,000
Long-term debt maturities repayment of principal in year two	13,600,000
Long-term debt maturities repayment of principal in year three	25,100,000
Long-term debt maturities repayment of principal in year four	20,600,000
Long-term debt maturities repayment of principal in year five	17,600,000
Long-term debt maturities repayment of principal in year five thereafter	364,100,000
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	$ 10,000,000.0
Cumulative consolidated net income base	509,500,000
Cumulative net income with restrictions	227,500,000
Cumulative net income free of restrictions	282,000,000.0
Restricted net assets of consolidated subsidiaries	$ 38,800,000
Percentage of restricted net assets	6.92%
Long-term debt including current maturities	$ 486,600,000	$ 327,200,000
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	3.23%
Long-term debt including current maturities	$ 0	30,000,000
Uncollateralized Senior Note Due On Two Thousand Twenty Six [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	5.68%
Long-term debt including current maturities	$ 20,300,000	23,200,000
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	$ 168,100,000
Cumulative net income with restrictions	(37,600,000)
Cumulative net income free of restrictions	130,500,000
Long-term debt including current maturities	$ 7,990,000	7,986,000
Uncollateralized Senior Note Due December Two Thousand Thirty Four [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	2.98%
Long-term debt including current maturities	$ 70,000,000
Uncollateralized Senior Note Due On May 2 Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	6.43%
Long-term debt including current maturities	$ 6,300,000	7,000,000
Uncollateralized Senior Note Two Due on December Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	3.73%
Long-term debt including current maturities	$ 18,000,000	20,000,000
Uncollateralized Senior Notes Due On Two Thousand Twenty Nine [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	3.88%
Long-term debt including current maturities	$ 50,000,000	50,000,000
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%
Long-term debt including current maturities	$ 2,000,000	4,000,000
Uncollateralized Senior Note Due On Two Thousand Twenty Three [Member]
Debt Instrument [Line Items]
Long-term debt, interest percentage	5.93%
Long-term debt including current maturities	$ 12,000,000	$ 15,000,000
Florida Public Utilities Company [Member]
Debt Instrument [Line Items]
Cumulative net income with restrictions	$ 2,500,000