Long-Term Debt - Outstanding Long-Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt	$ 205,200	$ 145,900
Capital lease obligation	2,070	3,471
Less: debt issuance costs	(433)	(291)
Long-term Debt and Capital Lease Obligations	206,816	149,053
Less: current maturities	(9,421)	(12,099)
Total long-term debt, net of current maturities	197,395	136,954
9.08% bond, due June 1, 2022
Debt Instrument [Line Items]
Long-term Debt	7,982	7,978
6.64% note, due October 31, 2017
Debt Instrument [Line Items]
Long-term Debt	0	2,727
5.50% note, due October 12, 2020
Debt Instrument [Line Items]
Long-term Debt	6,000	8,000
5.93% note, due October 31, 2023
Debt Instrument [Line Items]
Long-term Debt	18,000	21,000
5.68% note, due June 30, 2026
Debt Instrument [Line Items]
Long-term Debt	26,100	29,000
6.43% note, due May 2, 2028
Debt Instrument [Line Items]
Long-term Debt	7,000	7,000
3.73% note, due December 16, 2028
Debt Instrument [Line Items]
Long-term Debt	20,000	20,000
3.88% note, due May 15, 2029
Debt Instrument [Line Items]
Long-term Debt	50,000	50,000
3.25% note, due April 30, 2032 [Member]
Debt Instrument [Line Items]
Long-term Debt	70,000	0
Promissory note
Debt Instrument [Line Items]
Long-term Debt	$ 97	$ 168