Commitment and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Commitments And Contingencies [Line Items]
Reserve for unpaid and incurred but not reported claims, amount	$ 127,937	$ 122,598
Reserve for unpaid and incurred but not reported claims, classified in current liabilities	34,537	56,079
Reserve for unpaid and incurred but not reported claims, classified in non-current liabilities	93,400	66,519
Claims recoverable from third party insurance carriers	37,839	37,020
Other Current Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	9,365	17,462
Other Assets
Commitments And Contingencies [Line Items]
Claims recoverable from third party insurance carriers	$ 28,474	$ 19,558