Commitment and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Commitments And Contingencies [Line Items]
Reserve for unpaid and incurred but not reported claims, amount	$ 126,392	$ 127,937
Reserve for unpaid and incurred but not reported claims, classified in current liabilities	43,948	34,537
Reserve for unpaid and incurred but not reported claims, classified in non-current liabilities	82,444	$ 93,400
Claims recoverable from third party insurance carriers	36,606
Other Current Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	8,659
Other Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	$ 27,947