Long-Term Debt and Liquidity Matters - Estimated Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Estimated fair value of long-term debt, including current maturities
Carrying Amount	$ 5,087,364	$ 4,871,713
Fair Value	5,192,395	5,304,956
Pinnacle West
Estimated fair value of long-term debt, including current maturities
Carrying Amount	298,640	298,421
Fair Value	292,185	298,608
APS
Estimated fair value of long-term debt, including current maturities
Carrying Amount	4,788,724	4,573,292
Fair Value	$ 4,900,210	$ 5,006,348