Debt (Recourse Debt) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Long-term debt
Less: current maturities	$ 75,475	$ 62,899
Collateralized debt book value	172,400
Recourse
Long-term debt
Long-term debt, gross	847,650	747,184
Less: current maturities	69,037	53,353
Less: unamortized prepaid debt issuance costs	5,479	7,833
Long-term debt, less current maturities	773,134	685,998
Recourse | Note payable, variable rate (1.85% at December 31, 2020), payable in increasing amounts plus interest, due 2026
Long-term debt
Long-term debt, gross	$ 225,000	237,500
Interest rate of debt instruments	1.85%
Recourse | Note payable, variable rate (1.60% at December 31, 2020), payable in increasing amounts plus interest, due 2026
Long-term debt
Long-term debt, gross	$ 150,000	0
Interest rate of debt instruments	1.60%
Recourse | Note payable, variable rate (1.60% at December 31, 2020), payable in increasing amounts plus interest, due 2024
Long-term debt
Long-term debt, gross	$ 145,000	157,500
Interest rate of debt instruments	1.60%
Recourse | Note payable, 4.50%, payable at maturity, due 2034 (a)
Long-term debt
Long-term debt, gross	$ 102,524	105,000
Interest rate of debt instruments	4.50%
Recourse | Finance lease obligations, due serially to 2030 (a)
Long-term debt
Long-term debt, gross	$ 36,484	38,482
Recourse | Note payable, 4.07%, payable at maturity, due 2021
Long-term debt
Long-term debt, gross	$ 26,000	26,000
Interest rate of debt instruments	4.07%
Recourse | Note payable, 4.85%, payable at maturity, due 2026
Long-term debt
Long-term debt, gross	$ 25,000	25,000
Interest rate of debt instruments	4.85%
Recourse | Note payable, 4.55%, payable at maturity, due 2023
Long-term debt
Long-term debt, gross	$ 24,000	24,000
Interest rate of debt instruments	4.55%
Recourse | Note payable, 3.33%, payable in increasing amounts plus interest, due 2025 (a)
Long-term debt
Long-term debt, gross	$ 22,634	23,780
Interest rate of debt instruments	3.33%
Recourse | Note payable, 3.29%, payable in increasing amounts plus interest, due 2022 (a)
Long-term debt
Long-term debt, gross	$ 16,028	17,497
Interest rate of debt instruments	3.29%
Recourse | Note payable, 4.50%, payable at maturity, due 2030
Long-term debt
Long-term debt, gross	$ 16,000	16,000
Interest rate of debt instruments	4.50%
Recourse | Note payable, 5.00%, payable at maturity, due 2040
Long-term debt
Long-term debt, gross	$ 14,000	14,000
Interest rate of debt instruments	5.00%
Recourse | Note payable, variable rate (1.77% at December 31, 2020), payable quarterly, due 2024 (a)
Long-term debt
Long-term debt, gross	$ 11,250	12,250
Interest rate of debt instruments	1.77%
Recourse | Industrial revenue bond, variable rate (1.88% at December 31, 2020), payable at maturity, due 2036
Long-term debt
Long-term debt, gross	$ 21,000	21,000
Interest rate of debt instruments	1.88%
Recourse | Debenture bonds, 2.65% to 4.50%, payable in increasing amounts plus interest, due 2021 through 2031
Long-term debt
Long-term debt, gross	$ 12,730	26,075
Recourse | Debenture bonds, 2.65% to 4.50%, payable in increasing amounts plus interest, due 2021 through 2031 | Minimum
Long-term debt
Interest rate of debt instruments	2.65%
Recourse | Debenture bonds, 2.65% to 4.50%, payable in increasing amounts plus interest, due 2021 through 2031 | Maximum
Long-term debt
Interest rate of debt instruments	4.50%
Recourse | Industrial revenue bond, variable rate, paid 2020
Long-term debt
Long-term debt, gross	$ 0	$ 3,100