Commitments And Contingencies (Details) (USD $)	Mar. 31, 2012
Credit Agreement [Member]
Commitments and Contingencies [Line Items]
Letters of credit outstanding	$ 33,554,000
Current Investments [Member]
Commitments and Contingencies [Line Items]
Investments providing collateral for letters of credit to guarantee insurance claims	31,584,000
Non-Current Investments [Member]
Commitments and Contingencies [Line Items]
Investments providing collateral for letters of credit to guarantee insurance claims	18,578,000
Guarantee Payment Of Insurance Claims [Member]
Commitments and Contingencies [Line Items]
Letters of credit outstanding	$ 45,146,000