Debt (Details 2) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Long-term debt
Total current maturities of long-term debt		$ 27,786	$ 76,415
Total long-term debt, less current maturities		436,208	298,638
Collateralized debt book value		208,400
Recourse
Long-term debt
Total long-term debt		463,994	375,053
Total current maturities of long-term debt		27,786	76,415
Total long-term debt, less current maturities		436,208	298,638
Nonrecourse
Long-term debt
Total current maturities of long-term debt		0	0
Total long-term debt, less current maturities		0	0
Senior note payable, 6.12%, payable at maturity, due 2015 | Recourse
Long-term debt
Total long-term debt		$ 0	61,500
Interest rate of debt instruments		6.12%
Senior note payable, 3.72%, payable at maturity, due 2017 | Recourse
Long-term debt
Total long-term debt		$ 25,000	25,000
Interest rate of debt instruments		3.72%
Senior note payable, 6.78%, payable at maturity, due 2018 | Recourse
Long-term debt
Total long-term debt		$ 41,500	41,500
Interest rate of debt instruments		6.78%
Note payable, 4.92%, $2 million annually ($2.2 million for 2015), plus interest, due 2021 | Recourse
Long-term debt
Total long-term debt	[1]	$ 22,666	24,911
Interest rate of debt instruments		4.92%
Annual payments		$ 2,000
Principal payments		2,200
Note payable, 4.76%, payable in increasing amounts ($1.9 million for 2015) plus interest, due 2028 | Recourse
Long-term debt
Total long-term debt	[1]	$ 49,949	51,818
Interest rate of debt instruments		4.76%
Principal payments		$ 1,900
Note payable, variable rate (2.74% at December 31, 2015), payable in increasing amounts ($1.3 million for 2015) plus interest, due 2023 | Recourse
Long-term debt
Total long-term debt	[1]	$ 20,513	21,791
Interest rate of debt instruments		2.74%
Principal payments		$ 1,300
Note payable, 3.29%, payable in increasing amounts ($1.2 million for 2015) plus interest, due 2022 | Recourse
Long-term debt
Total long-term debt	[1]	$ 22,913	24,159
Interest rate of debt instruments		3.29%
Principal payments		$ 1,200
Note payable, 4.23%, payable quarterly in varying amounts ($0.6 million for 2015) plus interest, due 2021 | Recourse
Long-term debt
Total long-term debt	[1]	$ 11,770	12,382
Interest rate of debt instruments		4.23%
Principal payments		$ 600
Notes payable, variable rate (1.45% at December 31, 2015), payable in varying amounts, ($0.0 million for 2015) plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt		$ 5,043	5,043
Interest rate of debt instruments		1.45%
Principal payments		$ 0
Note payable, variable rate (1.84% at December 31, 2015), payable in increasing amounts ($1.0 million for 2015) plus interest, due 2023 | Recourse
Long-term debt
Total long-term debt	[1]	$ 9,865	10,890
Interest rate of debt instruments		1.84%
Principal payments		$ 1,000
Note payable, variable rate (1.21% at December 31, 2015), $0.7 million annually, plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt	[1]	$ 7,350	8,050
Interest rate of debt instruments		1.21%
Principal payments		$ 700
Note payable, 4.76%, payable quarterly in varying amounts ($0.3 million for 2015) plus interest, due 2028 | Recourse
Long-term debt
Total long-term debt	[1]	$ 9,313	9,654
Interest rate of debt instruments		4.76%
Principal payments		$ 300
Note payable, 1.89%, payable in increasing amounts ($15.0 million for 2015) plus interest, due 2019 | Recourse
Long-term debt
Total long-term debt	[1]	$ 105,000	0
Interest rate of debt instruments		1.89%
Principal payments		$ 15,000
Note payable, 3.33%, payable in increasing amounts ($0.0 million for 2015) plus interest, due 2025 | Recourse
Long-term debt
Total long-term debt	[1]	$ 28,000	0
Interest rate of debt instruments		3.33%
Principal payments		$ 0
Note payable, 4.5%, payable at maturity, due 2030 | Recourse
Long-term debt
Total long-term debt	[1]	$ 16,000	0
Interest rate of debt instruments		4.50%
Note payable, 5.0%, payable at maturity, due 2040 | Recourse
Long-term debt
Total long-term debt	[1]	$ 14,000	0
Interest rate of debt instruments		5.00%
Variable rate (2.68% at December 31, 2015), payable at maturity, due 2017 | Recourse
Long-term debt
Total long-term debt	[1]	$ 6,987	7,461
Interest rate of debt instruments		2.68%
Variable rate (1.56% at December 31, 2015), payable at maturity, due 2019 | Recourse
Long-term debt
Total long-term debt	[1]	$ 4,650	4,650
Interest rate of debt instruments		1.56%
Variable rate (1.56% at December 31, 2015), payable at maturity, due 2025 | Recourse
Long-term debt
Total long-term debt	[1]	$ 3,100	3,100
Interest rate of debt instruments		1.56%
Variable rate (1.51% at December 31, 2015), payable at maturity, due 2036 | Recourse
Long-term debt
Total long-term debt	[1]	$ 21,000	21,000
Interest rate of debt instruments		1.51%
Debenture bonds, 2.65% to 5.00%, due 2016 through 2030 | Recourse
Long-term debt
Total long-term debt		$ 39,375	42,098
Debenture bonds, 2.65% to 5.00%, due 2016 through 2030 | Recourse | Minimum
Long-term debt
Interest rate of debt instruments		2.65%
Debenture bonds, 2.65% to 5.00%, due 2016 through 2030 | Recourse | Maximum
Long-term debt
Interest rate of debt instruments		5.00%
Other notes payable and bonds | Recourse
Long-term debt
Total long-term debt		$ 0	$ 46

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $208.4 million