Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt
Total debt	$ 182,402,000	$ 199,454,000
Less current maturities	32,838,000	34,487,000
Long-term debt	149,564,000	164,967,000
Unsecured private placement notes one [Member]
Debt
Debt instrument interest rate percentage	4.86%
Maturity Dates	2017-2023
Total debt	65,000,000	65,000,000
Unsecured private placement notes two [Member]
Debt
Debt instrument interest rate percentage	5.88%
Maturity Dates	2016-2022
Total debt	40,000,000	40,000,000
Unsecured private placement notes three [Member]
Debt
Debt instrument interest rate percentage	5.69%
Maturity Dates	2013-2018
Total debt	34,286,000	40,000,000
Unsecured private placement notes four [Member]
Debt
Debt instrument interest rate percentage	6.86%
Maturity Dates	2013-2015
Total debt	12,856,000	17,142,000
Unsecured private placement notes five [Member]
Debt
Debt instrument interest rate percentage	6.59%
Total debt		2,727,000
Debt of foreign subsidiaries Secured bank term loans, foreign currency [Member]
Debt
Maturity Dates	2013-2015
Carrying value of variable interest secured term	9,531,000	12,496,000
Debt of foreign subsidiaries Secured bank term loan, U.S. dollars [Member]
Debt
Maturity Dates	2013-2014
Carrying value of variable interest secured term	3,500,000	5,833,000
Debt of foreign subsidiaries Other loans, foreign currency [Member]
Debt
Maturity Dates	2013-2015
Total debt	$ 17,229,000	$ 16,256,000