Debt (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Debt
Total debt	$ 195,343	$ 199,454
Less current maturities	33,294	34,487
Long-term debt	162,049	164,967
Unsecured private placement notes one [Member]
Debt
Debt instrument interest rate percentage	4.86%
Maturity Dates	2017-2023
Total debt	65,000	65,000
Unsecured private placement notes two [Member]
Debt
Debt instrument interest rate percentage	5.88%
Maturity Dates	2016-2022
Total debt	40,000	40,000
Unsecured private placement notes three [Member]
Debt
Debt instrument interest rate percentage	5.69%
Maturity Dates	2012-2018
Total debt	40,000	40,000
Unsecured private placement notes four [Member]
Debt
Debt instrument interest rate percentage	6.86%
Maturity Dates	2012-2015
Total debt	17,142	17,142
Unsecured private placement notes five [Member]
Debt
Debt instrument interest rate percentage	6.59%
Maturity Dates	2012-2012
Total debt	2,727	2,727
Debt of foreign subsidiaries Secured bank term loans, foreign currency [Member]
Debt
Maturity Dates	2012-2016
Carrying value of variable interest secured term	10,688	12,496
Debt of foreign subsidiaries Secured bank term loan, U.S. dollars [Member]
Debt
Maturity Dates	2012-2014
Carrying value of variable interest secured term	4,667	5,833
Debt of foreign subsidiaries Other loans, foreign currency [Member]
Debt
Maturity Dates	2012-2015
Total debt	$ 15,119	$ 16,256