Commitments and Contingencies (Details) - USD ($)	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
CPG and PNG COAs
Commitments and Contingencies
Option to termination agreement by either party effective at end of any two-year period beginning with the original effective date in March 2004 (in years)	2 years
Accrual for environmental loss contingencies	$ 11,679,000	$ 11,233,000
Environmental Issue
Commitments and Contingencies
Base year for determination of investigation and remediation cost (in years)	5 years
CPG MGP
Commitments and Contingencies
Environmental remediation expense	$ 1,800,000
PNG MGP
Commitments and Contingencies
Environmental remediation expense	$ 1,100,000