Long-Term Debt - Outstanding Long-Term Debt (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total long-term debt		$ (522,500)	$ (523,000)
Debt Instrument, Unused Borrowing Capacity, Amount		310,000
Total Long-term debt		521,665	522,099
Less: current maturities		(16,206)	(13,600)
Less: debt issuance costs		825	901
Total long-term debt, net of current maturities		505,459	508,499
Aggregated Unfunded Commitments [Member]
Debt Instrument [Line Items]
Total long-term debt		0
New York Life [Member]
Debt Instrument [Line Items]
Total long-term debt	[1]	$ (140,000)
9.08% bond, due June 1, 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		9.08%
5.50% note, due October 12, 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.50%
5.93% note, due October 31, 2023 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (7,500)	(9,000)
Debt Instrument, Interest Rate, Stated Percentage		5.93%
5.68% note, due June 30, 2026 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (14,500)	(17,400)
Debt Instrument, Interest Rate, Stated Percentage		5.68%
6.43% note, due May 2, 2028 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (4,900)	(5,600)
Debt Instrument, Interest Rate, Stated Percentage		6.43%
3.73% note, due December 16, 2028 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (16,000)	(16,000)
Debt Instrument, Interest Rate, Stated Percentage		3.73%
3.88% note, due May 15, 2029 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (40,000)	(45,000)
Debt Instrument, Interest Rate, Stated Percentage		3.88%
3.25% note, due April 30, 2032 [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (70,000)	(70,000)
3.48% note, due May 31, 2038 [Member]
Debt Instrument [Line Items]
Total long-term debt		(50,000)	(50,000)
Uncollateralized Senior Note Due November Two Thousand Thirty Eight [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (50,000)	(50,000)
Debt Instrument, Interest Rate, Stated Percentage		3.58%
Uncollateralized Senior Note Due July Two Thousand Thirty Five
Debt Instrument [Line Items]
Total long-term debt		$ (50,000)	(50,000)
Debt Instrument, Interest Rate, Stated Percentage		3.00%
Uncollateralized Senior Note Due August Two Thousand Thirty Five
Debt Instrument [Line Items]
Total long-term debt		$ (40,000)	(40,000)
Debt Instrument, Interest Rate, Stated Percentage		2.96%
Uncollateralized Senior Note Due November Two Thousand Thirty Nine
Debt Instrument [Line Items]
Total long-term debt		$ (100,000)	$ (100,000)
Equipment Security Note
Debt Instrument [Line Items]
Total long-term debt		$ (9,590)
Debt Instrument, Interest Rate, Stated Percentage		2.46%
Equipment Security Note | Marlin Gas Services [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (9,600)
Note Purchase Agreement
Debt Instrument [Line Items]
Total long-term debt		$ (50,000)
Debt Instrument, Interest Rate, Stated Percentage		2.49%
Aggregate Shelf Agreements [Member]
Debt Instrument [Line Items]
Total long-term debt		$ (360,000)
Aggregate Shelf Agreements [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		670,000
Prudential [Member]
Debt Instrument [Line Items]
Total long-term debt	[1]	(220,000)
Debt Instrument, Unused Borrowing Capacity, Amount	[1]	150,000
Prudential [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	[1]	370,000
MetLife [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	[1]	150,000
Debt Instrument, Unused Borrowing Capacity, Amount	[1]	$ 150,000

[1] (1) The Prudential, MetLife and NYL Shelf Agreements expire in April 2023, May 2023 and November 2021, respectively.