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, 2018	Sep. 30, 2017	Sep. 30, 2016
Assets (liabilities) recorded in the balance sheet:
Unfunded liabilities – included in other noncurrent liabilities	$ (81,590)	$ (143,674)
Pension Benefits
Change in benefit obligations:
Benefit obligations — beginning of year	639,245	645,444
Service cost	8,469	9,038
Interest cost	25,358	24,394
Actuarial gain	(36,050)	(14,575)
Benefits paid	(25,831)	(25,056)
Benefit obligations — end of year	611,191	639,245	$ 645,444
Change in plan assets:
Fair value of plan assets — beginning of year	498,080	463,432
Actual gain on assets	44,408	48,309
Employer contributions	15,079	11,395	9,869
Benefits paid	(25,831)	(25,056)
Fair value of plan assets — end of year	531,736	498,080	463,432
Funded status of the plans — end of year	(79,455)	(141,165)
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	(79,455)	(141,165)
Net amount recognized	(79,455)	(141,165)
Amounts recorded in stockholder’s equity (pre-tax):
Prior service cost (benefit)	80	105
Net actuarial loss (gain)	9,490	15,106
Total	9,570	15,211
Amounts recorded in regulatory assets and liabilities (pre-tax):
Prior service cost (benefit)	720	970
Net actuarial loss (gain)	85,746	139,505
Total	86,466	140,475
Other Postretirement Benefits
Change in benefit obligations:
Benefit obligations — beginning of year	11,904	12,075
Service cost	147	303
Interest cost	448	460
Actuarial gain	(1,348)	(512)
Benefits paid	(467)	(422)
Benefit obligations — end of year	10,684	11,904	12,075
Change in plan assets:
Fair value of plan assets — beginning of year	14,771	13,715
Actual gain on assets	913	1,333
Employer contributions	0	85
Benefits paid	(335)	(362)
Fair value of plan assets — end of year	15,349	14,771	$ 13,715
Funded status of the plans — end of year	4,665	2,867
Assets (liabilities) recorded in the balance sheet:
Assets in excess of liabilities – included in other noncurrent assets	6,729	5,382
Unfunded liabilities – included in other noncurrent liabilities	(2,064)	(2,514)
Net amount recognized	4,665	2,868
Amounts recorded in stockholder’s equity (pre-tax):
Prior service cost (benefit)	(12)	(23)
Net actuarial loss (gain)	(368)	(46)
Total	(380)	(69)
Amounts recorded in regulatory assets and liabilities (pre-tax):
Prior service cost (benefit)	(1,163)	(1,605)
Net actuarial loss (gain)	(135)	1,192
Total	$ (1,298)	$ (413)