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- Definition

Fixed price to acquire leased interest in facilities.

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No definition available.

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Generating Capacity Purchased, in MWs

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- Definition

Increase in Capital Lease Obligation due to Purchase Commitment

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- Definition

Increase in Utility Plant under Capital Lease due to Purchase Commitment

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- Definition

Jointly Owned, Percentage of Interest Committed to Purchase

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- Definition

Lease Arrangement, Fair Market Value Purchase Price

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- Definition

Lessee Leasing Arrangements, Operating Leases, Term of Contract, Renewal

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Lessee Leasing Arrangements,Term of Contract, Renewal Percentage

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- Definition

Number of Leases

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No definition available.

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- Definition

Number of Leases Remaining

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- Definition

Percentage of ownership in generating units.

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No definition available.

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- Definition

Sales Price of Leased Interest In Facilities

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No definition available.

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- Definition

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

+ References

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- Definition

Amount of capital lease obligation due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19
-Article 5

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- Definition

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- 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.

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No definition available.

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- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

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No definition available.

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- Definition

Fixed interest rate related to the interest rate derivative.

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- Definition

Nominal or face amount used to calculate payments on the derivative liability.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Nominal or face amount used to calculate payments on a nonderivative instrument.

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No definition available.

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DEBT, CREDIT FACILITY, AND CAPITAL LEASE OBLIGATIONS (Capital Lease) (Details)	1 Months Ended		12 Months Ended			18 Months Ended
	Dec. 31, 2016 USD ($) lease	Apr. 30, 2015 USD ($)	Dec. 31, 2016 USD ($) lease	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Nov. 30, 2016 USD ($)	Sep. 16, 2016	Jan. 31, 2016 MW	May 31, 2015 USD ($)	Jan. 31, 2015 USD ($) MW
Debt Instrument [Line Items]
Capital Lease Obligations	$ 91,000,000		$ 91,000,000	$ 69,000,000
Current Obligations Under Capital Leases	51,765,000		51,765,000	14,114,000
Capital Lease Obligations, Noncurrent	39,267,000		39,267,000	55,324,000
Depreciation			146,097,000	138,093,000	$ 126,520,000
Commitment to Purchase Capital Lease Interests			36,000,000	$ 0	$ 109,000,000
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Notional amount	$ 23,000,000		$ 23,000,000
Springerville Common Facilities Lease Debt [Member] \| Interest Rate Swap [Member]
Debt Instrument [Line Items]
Fixed rate of interest related to interest rate swap	5.77%		5.77%
Notional amount	$ 23,000,000		$ 23,000,000
Notional amount of debt	$ 9,000,000
Derivative basis spread	1.88%		1.88%	1.88%
Springerville Common Facility Lease Expiring December 2017 [Member]
Debt Instrument [Line Items]
Fixed price to acquire leased interest in facilities			$ 38,000,000
Number of operating lease \| lease	1		1
Springerville Common Facility Lease Expiring January 2021[Member]
Debt Instrument [Line Items]
Fixed price to acquire leased interest in facilities			$ 68,000,000
Number of operating lease \| lease	2		2
Number of Leases Remaining \| lease	2		2
Term of contract, renewal			2 years
Springerville Common Facility Lease Expiring January 2021[Member] \| SRP [Member]
Debt Instrument [Line Items]
Lessee Leasing Arrangements,Term of Contract, Renewal Percentage	14.00%		14.00%
Springerville Common Facility Lease Expiring January 2021[Member] \| Tri-State [Member]
Debt Instrument [Line Items]
Lessee Leasing Arrangements,Term of Contract, Renewal Percentage	14.00%		14.00%
Springerville Common Facilities [Member]
Debt Instrument [Line Items]
Percentage of interest committed to purchase	17.80%		17.80%
Springerville Common Facilities [Member] \| Springerville Common Facility Lease Expiring December 2017 [Member]
Debt Instrument [Line Items]
Commitment to Purchase Capital Lease Interests	$ 38,000,000
Increase in Utility Plant under Capital Lease	36,000,000
Capital Lease Obligations Incurred	$ 36,000,000
Springerville Unit One [Member] \| Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating station										24.80%
Generating capacity purchased, in MWs \| MW										96
Springerville Unit One [Member] \| Completion of Purchase of Equity Interest [Member]
Debt Instrument [Line Items]
Percentage of ownership in generating station								49.50%
Generating capacity purchased, in MWs \| MW								192
Percentage Of Ownership In Generating Units							100.00%
Springerville Unit One [Member] \| Springerville Unit One Lease [Member] \| Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Lease arrangement, fair market value purchase price										$ 46,000,000
Springerville Coal Handling Facilities
Debt Instrument [Line Items]
Percentage of ownership in generating station	83.00%		83.00%
Springerville Coal Handling Facilities \| SRP [Member]
Debt Instrument [Line Items]
Percentage of ownership in generating station									17.05%
Springerville Coal Handling Facilities \| Tri-State [Member]
Debt Instrument [Line Items]
Percentage of ownership in generating station									17.05%
Depreciation						$ 1,000,000
Springerville Coal Handling Facilities \| Tri-State [Member] \| Assets Held under Capital Leases [Member]
Debt Instrument [Line Items]
Percentage of ownership in generating station				17.05%
Springerville Coal Handling Facilities \| Tri-State [Member] \| Electricity Generation Plant, Non-Nuclear [Member]
Debt Instrument [Line Items]
Percentage of ownership in generating station	17.05%		17.05%
Springerville Coal Handling Facilities \| Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Percentage of ownership in generating station		86.70%
Springerville Coal Handling Facilities \| Completion of Purchase of Equity Interest [Member]
Debt Instrument [Line Items]
Percentage Of Ownership In Generating Units		100.00%
Springerville Coal Handling Facilities \| Springerville Coal Handling Facilities Lease [Member] \| SRP [Member]
Debt Instrument [Line Items]
Sales price of leased interest in facilities									$ 24,000,000
Springerville Coal Handling Facilities \| Springerville Coal Handling Facilities Lease [Member] \| Tri-State [Member]
Debt Instrument [Line Items]
Sales price of leased interest in facilities									$ 24,000,000
Springerville Coal Handling Facilities \| Springerville Coal Handling Facilities Lease [Member] \| Additional Purchase of Equity Interest
Debt Instrument [Line Items]
Fixed price to acquire leased interest in facilities		$ 120,000,000