Debt (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Long-term debt
Total current maturities of long-term debt	$ 15,145,000		$ 32,208,000
Total long-term debt, less current maturities	427,243,000		238,885,000
Collateralized debt book value	141,900,000
Recourse
Long-term debt
Total long-term debt	419,825,000		270,139,000
Total current maturities of long-term debt	12,649,000		32,051,000
Total long-term debt, less current maturities	407,176,000		238,088,000
Nonrecourse
Long-term debt
Total long-term debt	22,563,000		954,000
Total current maturities of long-term debt	2,496,000		157,000
Total long-term debt, less current maturities	20,067,000		797,000
Senior note payable, 4.55%, payable at maturity, due 2012 | Recourse
Long-term debt
Total long-term debt	0		25,000,000
Interest rate of debt instruments	4.55%
Senior note payable, 5.52%, payable at maturity, due 2017
Long-term debt
Total long-term debt	25,000,000		25,000,000
Senior note payable, 5.52%, payable at maturity, due 2017 | Recourse
Long-term debt
Interest rate of debt instruments	5.52%
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, 6.00%, $2 million annually, plus interest, due 2021
Long-term debt
Total long-term debt	27,833,000	[1]	0	[1]
Note payable, 6.00%, $2 million annually, plus interest, due 2021 | Recourse
Long-term debt
Interest rate of debt instruments	6.00%
Principal payments	2,000,000
Note payable, 4.76%, payable in increasing amounts ($1.3 million for 2013) plus interest, due 2028
Long-term debt
Total long-term debt	55,300,000	[1]	0	[1]
Note payable, 4.76%, payable in increasing amounts ($1.3 million for 2013) plus interest, due 2028 | Recourse
Long-term debt
Interest rate of debt instruments	4.76%
Principal payments	1,300,000
Note payable, variable rate (2.71% at December 31, 2012), payable in increasing amounts ($1.1 million for 2013) plus interest, due 2023
Long-term debt
Total long-term debt	24,188,000	[1]	0	[1]
Note payable, variable rate (2.71% at December 31, 2012), payable in increasing amounts ($1.1 million for 2013) plus interest, due 2023 | Recourse
Long-term debt
Interest rate of debt instruments	2.71%
Principal payments	1,100,000
Note payable, 3.29%, payable in increasing amounts ($1.1 million for 2013) plus interest, due 2022
Long-term debt
Total long-term debt	26,533,000	[1]	0	[1]
Note payable, 3.29%, payable in increasing amounts ($1.1 million for 2013) plus interest, due 2022 | Recourse
Long-term debt
Interest rate of debt instruments	3.29%
Principal payments	1,200,000
Note payable, variable rate (1.94% at December 31, 2012), payable at maturity, due 2015
Long-term debt
Total long-term debt	25,000,000		0
Note payable, variable rate (1.94% at December 31, 2012), payable at maturity, due 2015 | Recourse
Long-term debt
Interest rate of debt instruments	1.94%
Note payable, variable rate (1.46% at December 31, 2012), payable $0.4 million monthly, plus interest, due 2015
Long-term debt
Total long-term debt	12,058,000		0
Note payable, variable rate (1.46% at December 31, 2012), payable $0.4 million monthly, plus interest, due 2015 | Recourse
Long-term debt
Interest rate of debt instruments	1.46%
Principal payments	400,000
Note payable, variable rate (1.71% at December 31, 2012), payable in increasing amounts ($1.0 million for 2013) plus interest, due 2023
Long-term debt
Total long-term debt	12,815,000	[1]	13,715,000	[1]
Note payable, variable rate (1.71% at December 31, 2012), payable in increasing amounts ($1.0 million for 2013) plus interest, due 2023 | Recourse
Long-term debt
Interest rate of debt instruments	1.71%
Principal payments	1,000,000
Note payable, variable rate (1.02% at December 31, 2012), $0.7 million annually, plus interest, due 2016
Long-term debt
Total long-term debt	9,450,000	[1]	10,150,000	[1]
Note payable, variable rate (1.02% at December 31, 2012), $0.7 million annually, plus interest, due 2016 | Recourse
Long-term debt
Interest rate of debt instruments	1.02%
Principal payments	700,000
Note payable, 8.5%, payable monthly in varying amounts ($0.1 million for 2013) plus interest, due 2016
Long-term debt
Total long-term debt	1,079,000	[1]	1,160,000	[1]
Note payable, 8.5%, payable monthly in varying amounts ($0.1 million for 2013) plus interest, due 2016 | Recourse
Long-term debt
Interest rate of debt instruments	8.50%
Principal payments	100,000
Variable rate (2.60% at December 31, 2012), due 2017
Long-term debt
Total long-term debt	8,408,000	[1]	8,881,000	[1]
Variable rate (2.60% at December 31, 2012), due 2017 | Recourse
Long-term debt
Interest rate of debt instruments	2.60%
Variable rate (2.08% at December 31, 2012), due 2019
Long-term debt
Total long-term debt	4,650,000	[1]	4,650,000	[1]
Variable rate (2.08% at December 31, 2012), due 2019 | Recourse
Long-term debt
Interest rate of debt instruments	2.08%
Variable rate (2.20% at December 31, 2012), due 2025
Long-term debt
Total long-term debt	3,100,000	[1]	3,100,000	[1]
Variable rate (2.20% at December 31, 2012), due 2025 | Recourse
Long-term debt
Interest rate of debt instruments	2.20%
Variable rate (1.89% at December 31, 2012), due 2036
Long-term debt
Total long-term debt	21,000,000	[1]	21,000,000	[1]
Variable rate (1.89% at December 31, 2012), due 2036 | Recourse
Long-term debt
Interest rate of debt instruments	1.89%
Debenture bonds, 2.65% to 5.00%, due 2014 through 2027
Long-term debt
Total long-term debt	35,411,000		29,483,000
Debenture bonds, 2.65% to 5.00%, due 2014 through 2027 | Recourse | Minimum
Long-term debt
Interest rate of debt instruments	2.65%
Debenture bonds, 2.65% to 5.00%, due 2014 through 2027 | Recourse | Maximum
Long-term debt
Interest rate of debt instruments	5.00%
Note payable, variable rate (3.71% at December 31, 2012), payable at maturity, due 2022 | Nonrecourse
Long-term debt
Total long-term debt	9,378,000		0
Interest rate of debt instruments	3.71%
Note payable, variable rate (3.71% at December 31, 2012), payable $3.1 million annually, due 2017 | Nonrecourse
Long-term debt
Total long-term debt	12,400,000		0
Interest rate of debt instruments	3.71%
Principal payments	3,100,000
Note payable, 6.37%, payable $24 monthly, due 2014 | Nonrecourse
Long-term debt
Total long-term debt	785,000		954,000
Interest rate of debt instruments	6.37%
Principal payments	$ 24,000

[1] Debt is collateralized by first mortgages on certain facilities and related equipment or other assets with a book value of $141.9 million