Debt (Details 2) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Long-term debt
Less: current maturities		$ 47,545	$ 27,786
Total long-term debt		397,065	436,208
Collateralized debt book value		179,300
Recourse
Long-term debt
Long-term Debt, Gross		447,823	463,994
Less: current maturities		47,545	27,786
Less: unamortized prepaid debt issuance costs		3,213	0
Total long-term debt		397,065	436,208
Nonrecourse
Long-term debt
Less: current maturities		0	0
Total long-term debt		0	0
Note payable, 4.07%, payable at maturity, due 2021 | Recourse
Long-term debt
Long-term Debt, Gross		$ 26,000	0
Interest rate of debt instruments		4.07%
Senior note payable, 3.72%, payable at maturity, due 2017 | Recourse
Long-term debt
Long-term Debt, Gross		$ 25,000	25,000
Interest rate of debt instruments		3.72%
Note payable, 4.55%, payable at maturity, due 2023 | Recourse
Long-term debt
Long-term Debt, Gross		$ 24,000	0
Interest rate of debt instruments		4.55%
Note payable, 4.85%, payable at maturity, due 2026 | Recourse
Long-term debt
Long-term Debt, Gross		$ 25,000	0
Interest rate of debt instruments		4.85%
Senior note payable, 6.78%, payable at maturity, due 2018 | Recourse
Long-term debt
Long-term Debt, Gross		$ 41,500	41,500
Interest rate of debt instruments		6.78%
Note payable, 4.92%, payable in increasing amounts ($2.2 million for 2016), plus interest, due 2021 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 20,443	22,666
Interest rate of debt instruments		4.92%
Annual payments		$ 2,000
Note payable, 4.76%, payable in increasing amounts ($2.0 million for 2016) plus interest, due 2028 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 47,990	49,949
Interest rate of debt instruments		4.76%
Annual payments		$ 2,000
Note payable, variable rate (3.12% at December 31, 2016), payable in increasing amounts ($1.3 million for 2016) plus interest, due 2023 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 19,179	20,513
Interest rate of debt instruments		3.12%
Annual payments		$ 1,000
Note payable, 3.29%, payable in increasing amounts ($1.3 million for 2016) plus interest, due 2022 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 21,619	22,913
Interest rate of debt instruments		3.29%
Annual payments		$ 1,000
Note payable, 4.23%, payable quarterly in varying amounts ($0.6 million for 2016) plus interest, due 2021 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 11,136	11,770
Interest rate of debt instruments		4.23%
Annual payments		$ 1,000
Notes payable, variable rate, due 2016 | Recourse
Long-term debt
Long-term Debt, Gross		0	5,043
Note payable, variable rate (2.21% at December 31, 2016), payable in increasing amounts ($1.1 million for 2016) plus interest, due 2023 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 8,790	9,865
Interest rate of debt instruments		1.84%
Annual payments		$ 1,000
Note payable, variable rate, due 2016 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	0	7,350
Note payable, variable rate (2.45% at December 31, 2016), payable in varying amounts ($0.1 million for 2016), plus interest, due 2026 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 9,016	0
Interest rate of debt instruments		2.45%
Annual payments		$ 0
Note payable, 4.76%, payable quarterly in varying amounts ($0.4 million for 2016) plus interest, due 2028 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 8,956	9,313
Interest rate of debt instruments		4.76%
Annual payments		$ 0
Note payable, 2.21%, payable in increasing amounts ($75.0 million for 2016) plus interest, due 2019 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 30,000	105,000
Interest rate of debt instruments		1.89%
Annual payments		$ 75,000
Note payable, 3.33%, payable in increasing amounts ($1.0 million for 2016) plus interest, due 2025 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 27,000	28,000
Interest rate of debt instruments		3.33%
Annual payments		$ 1,000
Note payable, 4.5%, payable at maturity, due 2030 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 16,000	16,000
Interest rate of debt instruments		4.50%
Note payable, 5.0%, payable at maturity, due 2040 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 14,000	14,000
Interest rate of debt instruments		5.00%
Variable rate (3.05% at December 31, 2016), payable at maturity, due 2017 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 6,513	6,987
Interest rate of debt instruments		2.68%
Variable rate (2.36% at December 31, 2016), payable at maturity, due 2019 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 4,650	4,650
Interest rate of debt instruments		1.56%
Variable rate (2.31% at December 31, 2016), payable at maturity, due 2025 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 3,100	3,100
Interest rate of debt instruments		1.56%
Variable rate (2.28% at December 31, 2016), payable at maturity, due 2036 | Recourse
Long-term debt
Long-term Debt, Gross	[1]	$ 21,000	21,000
Interest rate of debt instruments		1.51%
Debenture bonds, 2.65% to 5.00%, due 2017 through 2031 | Recourse
Long-term debt
Long-term Debt, Gross		$ 36,931	$ 39,375
Debenture bonds, 2.65% to 5.00%, due 2017 through 2031 | Recourse | Minimum
Long-term debt
Interest rate of debt instruments		2.65%
Debenture bonds, 2.65% to 5.00%, due 2017 through 2031 | Recourse | 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 $179.3 million