Defined Benefit Pension and Other Postretirement Plans - Schedule of Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,731	$ 1,741	$ 5,195	$ 5,222
Interest cost	5,818	5,628	17,453	16,883
Expected return on assets	(7,167)	(7,225)	(21,502)	(21,674)
Amortization of:
Prior service cost (benefit)	87	87	261	261
Actuarial loss	2,393	2,199	7,179	6,595
Net benefit cost (income)	2,862	2,430	8,586	7,287
Change in associated regulatory liabilities	0	0	0	0
Net benefit cost after change in regulatory liabilities	2,862	2,430	8,586	7,287
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	46	48	137	145
Interest cost	116	119	349	356
Expected return on assets	(149)	(153)	(447)	(459)
Amortization of:
Prior service cost (benefit)	(160)	(160)	(480)	(480)
Actuarial loss	24	32	73	95
Net benefit cost (income)	(123)	(114)	(368)	(343)
Change in associated regulatory liabilities	878	938	2,632	2,813
Net benefit cost after change in regulatory liabilities	$ 755	$ 824	$ 2,264	$ 2,470