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Debt and Other Obligations Debt and Other Obligations - Put and Call Options (Details) $ in Millions	Sep. 30, 2018 USD ($)
Debt Instrument
Amount of redeemable bond issues outstanding	$ 359
Call price	100.00%
Bond issues with survivor's option	219
PARRS 1998 Series D Bond
Debt Instrument
Amount of redeemable bond issues outstanding	$ 274
PARRS interest rate prior to rate reset	6.75%
PARRS interest rate after rate reset	3.55%
Amount of bonds redeemed	$ 301
PARRS 1999 Series A Bond
Debt Instrument
Amount of redeemable bond issues outstanding	$ 232
PARRS interest rate prior to rate reset	6.50%
PARRS interest rate after rate reset	3.36%
Amount of bonds redeemed	$ 293

X

- Definition

The amount of bonds redeemed.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of redeemable bond issues outstanding

+ References

No definition available.

+ Details

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Namespace Prefix:	tve_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The number of bond issues which include a "survivor's option" allowing for the right of redemption upon the death of a beneficial owner in certain specified circumstances.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Call price - low end of range. Shown as a percent of principal amount.

+ References

No definition available.

+ Details

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

X

- Definition

The PARRS interest rate after rate reset.

+ References

No definition available.

+ Details

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

X

- Definition

The PARRS interest rate prior to rate reset.

+ References

No definition available.

+ Details

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

X

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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