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		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,623	$ 2,053	$ 3,246	$ 4,105
Interest cost	5,721	5,196	11,442	10,391
Expected return on assets	(6,650)	(6,198)	(13,299)	(12,396)
Amortization of:
Prior service cost (benefit)	87	62	174	124
Actuarial loss	1,661	3,366	3,321	6,732
Net benefit cost (income)	2,442	4,479	4,884	8,956
Change in associated regulatory liabilities	0	0	0	0
Net expense	2,442	4,479	4,884	8,956
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure
Service cost	41	51	82	102
Interest cost	127	143	254	286
Expected return on assets	(139)	(132)	(278)	(264)
Amortization of:
Prior service cost (benefit)	(160)	(105)	(320)	(210)
Actuarial loss	37	91	74	182
Net benefit cost (income)	(94)	48	(188)	96
Change in associated regulatory liabilities	918	815	1,836	1,630
Net expense	$ 824	$ 863	$ 1,648	$ 1,726