Commitment and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Commitments And Contingencies [Line Items]
Reserve for unpaid and incurred but not reported claims, amount	$ 126,865	$ 127,937
Reserve for unpaid and incurred but not reported claims, classified in current liabilities	42,855	34,537
Reserve for unpaid and incurred but not reported claims, classified in non-current liabilities	84,010	$ 93,400
Claims recoverable from third party insurance carriers	35,697
Other Current Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	9,107
Other Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	$ 26,590