Debt (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Long-term debt
Total current maturities of long-term debt	$ 51,998,000		$ 15,145,000
Total long-term debt, less current maturities	375,213,000		427,243,000
Collateralized debt book value	151,700,000
Recourse
Long-term debt
Total long-term debt	417,136,000		419,825,000
Total current maturities of long-term debt	45,986,000		12,649,000
Total long-term debt, less current maturities	371,150,000		407,176,000
Nonrecourse
Long-term debt
Total long-term debt	10,075,000		22,563,000
Total current maturities of long-term debt	6,012,000		2,496,000
Total long-term debt, less current maturities	4,063,000		20,067,000
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, 6.10%, payable at maturity, due 2014 | Recourse
Long-term debt
Total long-term debt	25,000,000		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, 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	27,178,000	[1]	27,833,000	[1]
Interest rate of debt instruments	4.92%
Principal payments	2,455,000
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	53,600,000	[1]	55,300,000	[1]
Interest rate of debt instruments	4.76%
Principal payments	1,700,000
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	23,015,000	[1]	24,188,000	[1]
Interest rate of debt instruments	2.67%
Principal payments	1,172,000
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	25,366,000	[1]	26,533,000	[1]
Interest rate of debt instruments	3.29%
Principal payments	1,167,000
Line of credit, variable rate (1.87% at December 31, 2013), payable at maturity, due 2015 | Recourse
Long-term debt
Total long-term debt	0		25,000,000
Interest rate of debt instruments	1.87%
Notes payable, variable rate (1.42% at December 31, 2013), payable in varying amounts, (7.6 million for 2013) plus interest, due 2016 | Recourse
Long-term debt
Total long-term debt	22,120,000		12,058,000
Interest rate of debt instruments	1.42%
Principal payments	7,565,000
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	11,865,000	[1]	12,815,000	[1]
Interest rate of debt instruments	1.64%
Principal payments	950,000
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	8,750,000	[1]	9,450,000	[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	988,000	[1]	1,079,000	[1]
Interest rate of debt instruments	8.50%
Principal payments	90,000
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	9,980,000	[1]	0	[1]
Interest rate of debt instruments	4.76%
Principal payments	234,000
Note payable, 3.56%, payable monthly in varying amounts plus interest, due 2021 (a) | Recourse
Long-term debt
Total long-term debt	3,459,000	[1]	0	[1]
Interest rate of debt instruments	3.56%
Principal payments	0
Variable rate (2.55% at December 31, 2012), due 2017 | Recourse
Long-term debt
Total long-term debt	7,934,000	[1]	8,408,000	[1]
Interest rate of debt instruments	2.55%
Variable rate (1.97% at December 31, 2012), due 2019 | Recourse
Long-term debt
Total long-term debt	4,650,000	[1]	4,650,000	[1]
Interest rate of debt instruments	1.97%
Variable rate (2.10% at December 31, 2012), due 2025 | Recourse
Long-term debt
Total long-term debt	3,100,000	[1]	3,100,000	[1]
Interest rate of debt instruments	2.10%
Variable rate (1.81% at December 31, 2012), due 2036 | Recourse
Long-term debt
Total long-term debt	21,000,000	[1]	21,000,000	[1]
Interest rate of debt instruments	1.81%
Debenture bonds, 2.65% to 5.00%, due 2014 through 2028 | Recourse
Long-term debt
Total long-term debt	41,131,000		35,411,000
Debenture bonds, 2.65% to 5.00%, due 2014 through 2028 | Recourse | Minimum
Long-term debt
Interest rate of debt instruments	2.65%
Debenture bonds, 2.65% to 5.00%, due 2014 through 2028 | Recourse | Maximum
Long-term debt
Interest rate of debt instruments	5.00%
Line of credit, variable rate (3.92% at December 31, 2013), payable at maturity, due 2022 | Nonrecourse
Long-term debt
Total long-term debt	0		9,378,000
Interest rate of debt instruments	3.92%
Note payable, variable rate (3.92% at December 31, 2013), payable quarterly ($2.3 million for 2013) plus interest, due 2017 | Nonrecourse
Long-term debt
Total long-term debt	10,075,000		12,400,000
Interest rate of debt instruments	3.92%
Other notes payable | Nonrecourse
Long-term debt
Total long-term debt	$ 0		$ 785,000

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $151.7 million