PROPERTY, PLANT AND EQUIPMENT - Electric Plant (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Electric Plant
Electric plant in service (at cost)	$ 5,486,518	$ 5,193,236
Construction work in progress	216,279	206,097
Less allowances for depreciation and amortization	(2,240,732)	(2,129,173)
Net electric plant	3,462,065	3,270,160
Electric Plant
Electric Plant
Generation plant	3,498,248	3,379,305
Transmission plant	1,294,151	1,161,462
General plant	430,842	392,963
Other plant	263,277	259,506
Electric plant in service (at cost)	5,486,518	5,193,236
Construction work in progress	216,279	206,097
Less allowances for depreciation and amortization	(2,240,732)	(2,129,173)
Net electric plant	$ 3,462,065	$ 3,270,160
Generation plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	0.44%
Generation plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	3.16%
Transmission plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	2.00%
Transmission plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	2.88%
General plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	3.00%
General plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	33.33%
Other Plant \| Minimum
Electric Plant
Annual Depreciation Rate (as a percent)	2.80%
Other Plant \| Maximum
Electric Plant
Annual Depreciation Rate (as a percent)	5.60%

Period end amount of total gross PPE, excluding construction work in progress.

No definition available.

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- Definition

Period end book value of accumulated depreciation on property, plant and equipment (PPE) that is owned by the regulated operations of the public utility.

+ References

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- Definition

Period end amount of property, plant and equipment (PPE) related to regulated assets common to business units.

+ References

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Period end amount of construction work in progress in public utility.

+ References

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- Definition

Composite depreciation rate for public utility plants in service.

+ References

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- Definition

Period end amount of property, plant and equipment (PPE) related to generation or processing owned by public utility.

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- Definition

Period end amount of total net PPE.

+ References

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- Definition

Period end book value of other property, plant and equipment (PPE) owned (but not classified elsewhere) by the public utility.

+ References

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- Definition

Period end amount of property, plant and equipment (PPE) related to utility transmission owned by public utility.

+ References

No definition available.

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