Debt (Details 2) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Long-term debt
Less: current maturities		$ 54,205	$ 47,545
Total long-term debt		418,339	397,065
Collateralized debt book value		159,900
Recourse
Long-term debt
Long-term Debt, Gross		476,220	447,823
Less: current maturities		54,205	47,545
Less: unamortized prepaid debt issuance costs		3,676	3,213
Total long-term debt		418,339	397,065
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 | Notes payable, 3.72%, paid 2017
Long-term debt
Long-term Debt, Gross		$ 0	25,000
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, 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, 6.78%, payable at maturity, due 2018
Long-term debt
Long-term Debt, Gross		$ 41,500	41,500
Interest rate of debt instruments		6.78%
Recourse | Note payable, 4.92%, payable in increasing amounts ($2.2 million for 2017), plus interest, due 2021 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 18,241	20,443
Interest rate of debt instruments		4.92%
Annual payments		$ 2,200
Recourse | Note payable, 4.76%, payable in increasing amounts ($2.1 million for 2017) plus interest, due 2028 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 45,936	47,990
Interest rate of debt instruments		4.76%
Annual payments		$ 2,100
Recourse | Note payable, variable rate (3.86% at December 31, 2017), payable in increasing amounts ($1.4 million for 2017) plus interest, due 2023 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 17,786	19,179
Interest rate of debt instruments		3.86%
Annual payments		$ 1,400
Recourse | Note payable, 3.29%, payable in increasing amounts ($1.3 million for 2017) plus interest, due 2022 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 20,293	21,619
Interest rate of debt instruments		3.29%
Annual payments		$ 1,300
Recourse | Note payable, 4.23%, payable quarterly in varying amounts ($0.7 million for 2017) plus interest, due 2021 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 10,479	11,136
Interest rate of debt instruments		4.23%
Annual payments		$ 700
Recourse | Notes payable, variable rate, paid 2017
Long-term debt
Long-term Debt, Gross		0	8,790
Recourse | Note payable, variable rate (3.23% at December 31, 2017), payable in varying amounts ($0.3 million for 2017), plus interest, due 2026 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 8,762	9,016
Interest rate of debt instruments		3.23%
Annual payments		$ 300
Recourse | Note payable, 4.76%, payable quarterly in varying amounts ($0.4 million for 2017) plus interest, due 2028 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 8,581	8,956
Interest rate of debt instruments		4.76%
Annual payments		$ 400
Recourse | Note payable, 3.03%, payable at maturity, due 2022
Long-term debt
Long-term Debt, Gross		$ 100,000	30,000
Interest rate of debt instruments		3.03%
Recourse | Note payable, 3.33%, payable in increasing amounts ($1.0 million for 2017) plus interest, due 2025 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 25,960	27,000
Interest rate of debt instruments		3.33%
Annual payments		$ 1,000
Recourse | Note payable, 4.5%, 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, 5.0%, 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, paid 2017
Long-term debt
Long-term Debt, Gross		$ 0	6,513
Recourse | Variable rate (2.97% at December 31, 2017), payable at maturity, due 2024 (a)
Long-term debt
Long-term Debt, Gross		$ 14,500	0
Interest rate of debt instruments		2.97%
Recourse | Variable rate (3.33% at December 31, 2017), payable at maturity, due 2019 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 4,650	4,650
Interest rate of debt instruments		3.33%
Recourse | Variable rate (3.33% at December 31, 2017), payable at maturity, due 2025 (a)
Long-term debt
Long-term Debt, Gross	[1]	$ 3,100	3,100
Interest rate of debt instruments		3.33%
Recourse | Variable rate (3.25% at December 31, 2017), payable at maturity, due 2036
Long-term debt
Long-term Debt, Gross		$ 21,000	21,000
Interest rate of debt instruments		3.25%
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032
Long-term debt
Long-term Debt, Gross		$ 30,432	$ 36,931
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032 | Minimum
Long-term debt
Interest rate of debt instruments		2.65%
Recourse | Debenture bonds, 2.65% to 5.00%, due 2018 through 2032 | Maximum
Long-term debt
Interest rate of debt instruments		5.00%

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $159.9 million