Commitments and Contingencies (Details) (USD $)	Mar. 31, 2012
Notes payable to bank
Contingencies
Letters of credit for outstanding Industrial Development Revenue Bonds	$ 26,385,000
Letters of credit outstanding related to supply agreements	30,469,000
Letters of credit related to insurance coverages	21,892,000
Committed bank line
Contingencies
Letters of credit outstanding	78,850,000
Uncommitted bank lines
Contingencies
Letters of credit outstanding	14,011,000
Debt guarantees of non-consolidated affiliates and third party
Contingencies
Number of third parties to whom guarantees are outstanding	3
Maximum exposure for guarantees outstanding	$ 1,200,000