Commitment and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Reserve for unpaid and incurred but not reported claims, amount	$ 118,810	$ 122,598
Reserve for unpaid and incurred but not reported claims, classified in current liabilities	38,019	56,079
Reserve for unpaid and incurred but not reported claims, classified in non-current liabilities	80,791	$ 66,519
Claims recoverable from third party insurance carriers	30,797
Other Current Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	7,126
Other Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	$ 23,671