Debt (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Long-term debt
Total current maturities of long-term debt	$ 76,415,000		$ 51,998,000
Total long-term debt, less current maturities	298,638,000		375,213,000
Collateralized debt book value	123,100,000
Recourse
Long-term debt
Total long-term debt	375,053,000		417,136,000
Total current maturities of long-term debt	76,415,000		45,986,000
Total long-term debt, less current maturities	298,638,000		371,150,000
Nonrecourse
Long-term debt
Total current maturities of long-term debt	0		6,012,000
Total long-term debt, less current maturities	0		4,063,000
Senior note payable, 6.10%, payable at maturity, due 2014 | Recourse
Long-term debt
Total long-term debt	0		25,000,000
Interest rate of debt instruments	6.10%
Senior note payable, 6.12%, payable at maturity, due 2015 | Recourse
Long-term debt
Total long-term debt	61,500,000		61,500,000
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	25,000,000		25,000,000
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	41,500,000		41,500,000
Interest rate of debt instruments	6.78%
Note payable, 4.92%, $2 million annually ($2.5 million for 2013), plus interest, due 2021 | Recourse
Long-term debt
Total long-term debt	24,911,000	[1]	27,178,000	[1]
Interest rate of debt instruments	4.92%
Principal payments	2,266,000
Note payable, 4.76%, payable in increasing amounts ($1.7 million for 2013) plus interest, due 2028 | Recourse
Long-term debt
Total long-term debt	51,818,000	[1]	53,600,000	[1]
Interest rate of debt instruments	4.76%
Principal payments	1,782,000
Note payable, variable rate (2.67% at December 31, 2012), payable in increasing amounts ($1.2 million for 2013) plus interest, due 2023 | Recourse
Long-term debt
Total long-term debt	21,791,000	[1]	23,015,000	[1]
Interest rate of debt instruments	2.66%
Principal payments	1,224,000
Note payable, 3.29%, payable in increasing amounts ($1.2 million for 2013) plus interest, due 2022 | Recourse
Long-term debt
Total long-term debt	24,159,000	[1]	25,366,000	[1]
Interest rate of debt instruments	3.29%
Principal payments	1,207,000
Note payable, 4.23%, payable quarterly in varying amounts ($0.1 million for 2014) plus interest, due 2021 (a) | Recourse
Long-term debt
Total long-term debt	12,382,000	[1]	0	[1]
Interest rate of debt instruments	4.23%
Principal payments	150,000
Notes payable, variable rate (1.40% at December 31, 2014), payable in varying amounts, ($22.4 million for 2014) plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt	5,043,000		22,120,000
Interest rate of debt instruments	1.40%
Principal payments	22,386,000
Note payable, variable rate (1.64% at December 31, 2012), payable in increasing amounts ($1.0 million for 2013) plus interest, due 2023 | Recourse
Long-term debt
Total long-term debt	10,890,000	[1]	11,865,000	[1]
Interest rate of debt instruments	1.57%
Principal payments	975,000
Note payable, variable rate (.97% at December 31, 2012), $0.7 million annually, plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt	8,050,000	[1]	8,750,000	[1]
Interest rate of debt instruments	0.97%
Principal payments	700,000
Note payable, 8.5%, payable monthly in varying amounts ($0.1 million for 2013) plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt	0	[1]	988,000	[1]
Interest rate of debt instruments	8.50%
Principal payments	988,000
Note payable, 4.76%, payable quarterly in varying amounts ($0.2 million for 2013) plus interest, due 2028 | Recourse
Long-term debt
Total long-term debt	9,654,000	[1]	9,980,000	[1]
Interest rate of debt instruments	4.76%
Principal payments	325,000
Note payable, 3.56%, payable monthly in varying amounts plus interest, due 2021 (a) | Recourse
Long-term debt
Total long-term debt	0	[1]	3,459,000	[1]
Interest rate of debt instruments	3.56%
Principal payments	0
Variable rate (2.54% at December 31, 2014), payable at maturity, due 2017 (a) | Recourse
Long-term debt
Total long-term debt	7,461,000	[1]	7,934,000	[1]
Interest rate of debt instruments	2.54%
Variable rate (1.51% at December 31, 2014), payable at maturity, due 2019 (a) | Recourse
Long-term debt
Total long-term debt	4,650,000	[1]	4,650,000	[1]
Interest rate of debt instruments	1.51%
Variable rate (1.49% at December 31, 2014), payable at maturity, due 2025 (a) | Recourse
Long-term debt
Total long-term debt	3,100,000	[1]	3,100,000	[1]
Interest rate of debt instruments	1.49%
Variable rate (1.43% at December 31, 2014), payable at maturity, due 2036 (a) | Recourse
Long-term debt
Total long-term debt	21,000,000	[1]	21,000,000	[1]
Interest rate of debt instruments	1.43%
Debenture bonds, 2.65% to 5.00%, due 2015 through 2029 | Recourse
Long-term debt
Total long-term debt	42,098,000		41,131,000
Debenture bonds, 2.65% to 5.00%, due 2015 through 2029 | Recourse | Minimum
Long-term debt
Interest rate of debt instruments	2.65%
Debenture bonds, 2.65% to 5.00%, due 2015 through 2029 | 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	$ 46,000		$ 0

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $123.1 million