Long-Term Debt - Outstanding Long-Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Restricted Payment	$ 227,500
Long-term Debt	486,600	$ 327,200
Capital lease obligation	0	1,310
Less: debt issuance costs	(822)	(567)
Long-term Debt and Lease Obligation	485,768	327,955
Less: current maturities	(45,600)	(11,935)
Total long-term debt, net of current maturities	440,168	316,020
Unrestricted Payment	282,000
9.08% bond, due June 1, 2022
Debt Instrument [Line Items]
Restricted Payment	(37,600)
Long-term Debt	$ 7,990	7,986
Debt Instrument, Interest Rate, Stated Percentage	9.08%
Unrestricted Payment	$ 130,500
6.64% note, due October 31, 2017
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.64%
5.50% note, due October 12, 2020
Debt Instrument [Line Items]
Long-term Debt	$ 2,000	4,000
Debt Instrument, Interest Rate, Stated Percentage	5.50%
5.93% note, due October 31, 2023
Debt Instrument [Line Items]
Long-term Debt	$ 12,000	15,000
Debt Instrument, Interest Rate, Stated Percentage	5.93%
5.68% note, due June 30, 2026
Debt Instrument [Line Items]
Long-term Debt	$ 20,300	23,200
Debt Instrument, Interest Rate, Stated Percentage	5.68%
6.43% note, due May 2, 2028
Debt Instrument [Line Items]
Long-term Debt	$ 6,300	7,000
Debt Instrument, Interest Rate, Stated Percentage	6.43%
3.73% note, due December 16, 2028
Debt Instrument [Line Items]
Long-term Debt	$ 18,000	20,000
Debt Instrument, Interest Rate, Stated Percentage	3.73%
3.88% note, due May 15, 2029
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Debt Instrument, Interest Rate, Stated Percentage	3.88%
Uncollateralized Senior Note Due April Two Thousand Thirty Two [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 70,000	70,000
Uncollateralized Senior Note Due May Two Thousand Thirty Eight [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Debt Instrument, Interest Rate, Stated Percentage	3.48%
Uncollateralized Senior Note Due November Two Thousand Thirty Eight [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 50,000	50,000
Debt Instrument, Interest Rate, Stated Percentage	3.58%
Uncollateralized Senior Note Due August Two Thousand Thirty Nine [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	3.98%
Uncollateralized Senior Note Due December Two Thousand Thirty Four [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 70,000
Debt Instrument, Interest Rate, Stated Percentage	2.98%
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 0	30,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	0.75%
Debt Instrument, Interest Rate, Stated Percentage	3.23%
Term Note Due February Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 30,000
Debt Instrument, Interest Rate, Stated Percentage	2.46%
Promissory note
Debt Instrument [Line Items]
Long-term Debt	$ 0	$ 26
Electric Limited Proceedings [Member]
Debt Instrument [Line Items]
Basis Plus LIBOR for Term Notes to Finance Hurricane Michael Costs	75

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