Employee Retirement Plans - Change in Pension Benefits and Other Postretirement Benefit Obligations, Plan Assets, and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Assets (liabilities) recorded in the balance sheet:
Unfunded liabilities — included in other noncurrent liabilities	$ (184,516)	$ (135,003)
Pension Benefits
Change in benefit obligations:
Benefit obligations — beginning of year	563,621	539,725
Service cost	7,772	7,863
Interest cost	25,733	24,656
Actuarial loss (gain)	72,418	14,667
Benefits paid	(24,100)	(23,290)
Benefit obligations — end of year	645,444	563,621	$ 539,725
Change in plan assets:
Fair value of plan assets — beginning of year	430,789	442,465
Actual gain (loss) on assets	46,874	483
Employer contributions	9,869	11,131	19,227
Benefits paid	(24,100)	(23,290)
Fair value of plan assets — end of year	463,432	430,789	442,465
Funded status of the plans — end of year	(182,012)	(132,832)
Assets (liabilities) recorded in the balance sheet:
Assets in excess of liabilities — included in other noncurrent assets	0	0
Unfunded liabilities — included in other noncurrent liabilities	(182,012)	(132,832)
Net amount recognized	(182,012)	(132,832)
Amounts recorded in stockholder’s equity (pre-tax):
Prior service cost (credit)	138	178
Net actuarial loss (gain)	19,866	15,757
Total	20,004	15,935
Amounts recorded in regulatory assets and liabilities (pre-tax):
Prior service cost (credit)	1,262	1,570
Net actuarial loss	180,964	138,440
Total	182,226	140,010
Other Postretirement Benefits
Change in benefit obligations:
Benefit obligations — beginning of year	10,676	11,136
Service cost	198	220
Interest cost	483	511
Actuarial loss (gain)	1,117	(835)
Benefits paid	(399)	(356)
Benefit obligations — end of year	12,075	10,676	11,136
Change in plan assets:
Fair value of plan assets — beginning of year	12,523	12,848
Actual gain (loss) on assets	1,347	(95)
Employer contributions	98	126
Benefits paid	(253)	(356)
Fair value of plan assets — end of year	13,715	12,523	$ 12,848
Funded status of the plans — end of year	1,640	1,847
Assets (liabilities) recorded in the balance sheet:
Assets in excess of liabilities — included in other noncurrent assets	4,139	4,011
Unfunded liabilities — included in other noncurrent liabilities	(2,499)	(2,164)
Net amount recognized	1,640	1,847
Amounts recorded in stockholder’s equity (pre-tax):
Prior service cost (credit)	(35)	(48)
Net actuarial loss (gain)	(1)	(158)
Total	(36)	(206)
Amounts recorded in regulatory assets and liabilities (pre-tax):
Prior service cost (credit)	(2,247)	(2,890)
Net actuarial loss	2,425	2,289
Total	$ 178	$ (601)