Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Significant Accounting Policies [Line Items]
Cash, cash equivalents, and restricted cash		$ 37,326,000	$ 86,890,000	$ 139,188,000	$ 100,508,000
Cash and cash equivalents		37,076,000		139,188,000
Restricted cash		250,000
Investments, fair value		11,500,000
Accrued expenses and other current liabilities		107,304,000		103,855,000
Other long-term liabilities		26,772,000		25,255,000
Deferred charges and other assets, net		11,107,000		11,074,000
Long-term debt	[1]	$ 73,008,000		75,441,000
Operating lease expiration year		2032
Undiscounted future minimum lease payments through 2032		$ 122,500,000
New England Teamsters & Trucking Industry Pension Fund
Schedule Of Significant Accounting Policies [Line Items]
Accrued expenses and other current liabilities		200,000
Other long-term liabilities		17,400,000
Significant Other Observable Inputs Level 2 | New England Teamsters & Trucking Industry Pension Fund
Schedule Of Significant Accounting Policies [Line Items]
Multiemployer plan discounted withdrawal liability		21,200,000
Accounting Standards Update 2015-03
Schedule Of Significant Accounting Policies [Line Items]
Deferred charges and other assets, net				11,100,000
Long-term debt				75,400,000
Accounting Standards Update 2015-03 | Scenario, Previously Reported
Schedule Of Significant Accounting Policies [Line Items]
Deferred charges and other assets, net				11,900,000
Long-term debt				$ 76,300,000
Subsidiaries of Swiss Re
Schedule Of Significant Accounting Policies [Line Items]
Weather hedge contracts, payment entitled to be received per heating degree-day shortfall		$ 35,000
Percentage in heating degree days less than ten year average that is covered		92.50%
Derivative maximum payout		$ 12,500,000
Subsidiaries of Swiss Re | Delivery and branch expenses
Schedule Of Significant Accounting Policies [Line Items]
Credit on weather hedge contract		$ 0	$ 12,500,000

[1] Carrying amounts are net of unamortized debt issuance costs of $0.8 million as December 31, 2016 and $0.9 million as of September 30, 2016.