SCHEDULE II - RESERVE FOR UNCOLLECTIBLES (Details) - Reserve for uncollectibles. - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pinnacle West
Changes in reserve for uncollectibles
Balance at beginning of period	$ 3,037	$ 3,125	$ 3,094
Additions, Charged to cost and expenses	6,836	4,025	4,073
Additions, Charged to other accounts	0	0	0
Deductions	7,360	4,113	4,042
Balance at end of period	2,513	3,037	3,125
ARIZONA PUBLIC SERVICE COMPANY
Changes in reserve for uncollectibles
Balance at beginning of period	3,037	3,125	3,094
Additions, Charged to cost and expenses	6,836	4,025	4,073
Additions, Charged to other accounts	0	0	0
Deductions	7,360	4,113	4,042
Balance at end of period	$ 2,513	$ 3,037	$ 3,125

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

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Amount of increase in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from charges to costs and expenses.

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- Definition

Total of the deductions in a given period to allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs, representing receivables written off as uncollectible and portions of the reserves utilized, respectively.

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