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
	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,732	$ 1,741
Interest cost	5,817	5,627
Expected return on assets	(7,167)	(7,224)
Amortization of:
Prior service cost (benefit)	87	87
Actuarial loss	2,393	2,198
Net benefit cost (income)	2,862	2,429
Change in associated regulatory liabilities	0	0
Net benefit cost after change in regulatory liabilities	2,862	2,429
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	46	48
Interest cost	116	119
Expected return on assets	(149)	(153)
Amortization of:
Prior service cost (benefit)	(160)	(160)
Actuarial loss	24	32
Net benefit cost (income)	(123)	(114)
Change in associated regulatory liabilities	878	937
Net benefit cost after change in regulatory liabilities	$ 755	$ 823