Defined Benefit Pension and Other Postretirement Plans - Schedule of Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,741	$ 1,623
Interest cost	5,627	5,721
Expected return on assets	(7,224)	(6,650)
Amortization of:
Prior service cost (benefit)	87	87
Actuarial loss	2,198	1,661
Net benefit cost (income)	2,429	2,442
Change in associated regulatory liabilities	0	0
Net benefit cost after change in regulatory liabilities	2,429	2,442
VEBA
Defined Benefit Plan Disclosure
Service cost	48	41
Interest cost	119	127
Expected return on assets	(153)	(139)
Amortization of:
Prior service cost (benefit)	(160)	(160)
Actuarial loss	32	37
Net benefit cost (income)	(114)	(94)
Change in associated regulatory liabilities	937	918
Net benefit cost after change in regulatory liabilities	$ 823	$ 824