Benefit Plans Actuarial Assumptions (Details) - USD ($)	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure
Defined Benefit Plan, Cost of Living Adjustment Assumption	25.00%
Pension Benefits
Defined Benefit Plan Disclosure
Discount rate	3.20%	4.35%	3.85%
Discount rate	2.75%	3.20%	4.35%
Rate of compensation increase	3.43%	3.50%
Expected return on plan assets	6.75%	6.75%	6.75%
Defined Benefit Plan, Cost of Living Adjustment Assumption	2.00%	2.00%	2.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	3.43%	3.50%	5.34%
COLA percentage increase (decrease)	600.00%
Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Discount rate	3.30%	4.40%	3.95%
Discount rate	3.05%	3.30%
Defined Benefit Plan, Cost of Living Adjustment Assumption	2.00%	2.00%	2.00%
Minimum
Defined Benefit Plan Disclosure
Rate of compensation increase	2.50%
Maximum
Defined Benefit Plan Disclosure
Rate of compensation increase	14.00%
Pre-Medicare Eligible [Member] \| Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Current health care cost trend rate(2)	6.75%	6.25%	6.50%
Ultimate health care cost trend rate	5.00%	5.00%	5.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2027	2027
Initial health care cost trend rate	$ 0.0650	$ 0.0675
Post-Medicare Eligible [Member] [Member] \| Other Post-retirement Benefits
Defined Benefit Plan Disclosure
Current health care cost trend rate(2)	0.00%	0.00%	0.00%
Ultimate health care cost trend rate	4.00%	4.00%	4.00%
Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate	2024	2023

COLA percentage increase (decrease)

No definition available.

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Namespace Prefix:	tve_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The cost of living adjustment percentage used for defined benefit plans.

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Initial health care cost trend rate

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Year ultimate health care cost trend rate is expected to be reached, in CCYY format.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:gYearListItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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- Details

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- Details

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- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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- Details

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Namespace Prefix:
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Balance Type:
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- Details

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