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		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2,022	$ 1,732	$ 4,045	$ 3,464
Interest cost	5,539	5,818	11,078	11,635
Expected return on assets	(7,496)	(7,168)	(14,993)	(14,335)
Amortization of:
Prior service cost (benefit)	82	87	163	174
Actuarial loss	3,706	2,393	7,413	4,786
Net benefit cost (income)	3,853	2,862	7,706	5,724
Change in associated regulatory liabilities	0	0	0	0
Net benefit cost (income) after change in regulatory liabilities	3,853	2,862	7,706	5,724
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	61	45	122	91
Interest cost	107	117	215	233
Expected return on assets	(164)	(149)	(328)	(298)
Amortization of:
Prior service cost (benefit)	(160)	(160)	(320)	(320)
Actuarial loss	29	25	57	49
Net benefit cost (income)	(127)	(122)	(254)	(245)
Change in associated regulatory liabilities	(123)	876	(245)	1,754
Net benefit cost (income) after change in regulatory liabilities	$ (250)	$ 754	$ (499)	$ 1,509