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, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,732	$ 1,740	$ 3,464	$ 3,481
Interest cost	5,818	5,628	11,635	11,255
Expected return on assets	(7,168)	(7,225)	(14,335)	(14,449)
Amortization of:
Prior service cost (benefit)	87	87	174	174
Actuarial loss	2,393	2,198	4,786	4,396
Net benefit cost (income)	2,862	2,428	5,724	4,857
Change in associated regulatory liabilities	0	0	0	0
Net benefit cost after change in regulatory liabilities	2,862	2,428	5,724	4,857
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	45	49	91	97
Interest cost	117	118	233	237
Expected return on assets	(149)	(153)	(298)	(306)
Amortization of:
Prior service cost (benefit)	(160)	(160)	(320)	(320)
Actuarial loss	25	31	49	63
Net benefit cost (income)	(122)	(115)	(245)	(229)
Change in associated regulatory liabilities	876	938	1,754	1,875
Net benefit cost after change in regulatory liabilities	$ 754	$ 823	$ 1,509	$ 1,646