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, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2,023	$ 1,732
Interest cost	5,539	5,817
Expected return on assets	(7,497)	(7,167)
Amortization of:
Prior service cost (benefit)	81	87
Actuarial loss	3,707	2,393
Net benefit cost (income)	3,853	2,862
Change in associated regulatory liabilities	0	0
Net benefit cost (income) after change in regulatory liabilities	3,853	2,862
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	61	46
Interest cost	108	116
Expected return on assets	(164)	(149)
Amortization of:
Prior service cost (benefit)	(160)	(160)
Actuarial loss	28	24
Net benefit cost (income)	(127)	(123)
Change in associated regulatory liabilities	(122)	878
Net benefit cost (income) after change in regulatory liabilities	$ (249)	$ 755