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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2,023	$ 1,731	$ 6,068	$ 5,195
Interest cost	5,540	5,818	16,618	17,453
Expected return on assets	(7,497)	(7,167)	(22,490)	(21,502)
Amortization of:
Prior service cost (benefit)	81	87	244	261
Actuarial loss	3,706	2,393	11,119	7,179
Net benefit cost (income)	3,853	2,862	11,559	8,586
Change in associated regulatory liabilities	0	0	0	0
Net benefit cost (income) after change in regulatory liabilities	3,853	2,862	11,559	8,586
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	62	46	184	137
Interest cost	108	116	323	349
Expected return on assets	(164)	(149)	(492)	(447)
Amortization of:
Prior service cost (benefit)	(160)	(160)	(480)	(480)
Actuarial loss	28	24	85	73
Net benefit cost (income)	(126)	(123)	(380)	(368)
Change in associated regulatory liabilities	(123)	878	(368)	2,632
Net benefit cost (income) after change in regulatory liabilities	$ (249)	$ 755	$ (748)	$ 2,264