Retirement Plans and Other Benefits - Impact to Accumulated Other Comprehensive Loss (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Details related to accumulated other comprehensive loss
Accumulated other comprehensive loss		$ 0
Pension Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	$ 643,199,000	773,750,000
Prior service cost (credit)	0	81,000
APS’s portion recorded as a regulatory (asset) liability	(576,188,000)	(711,059,000)
Income tax expense (benefit)	(24,915,000)	(24,202,000)
Accumulated other comprehensive loss	42,096,000	38,570,000
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	28,334,000
Prior service credit	0
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2018	28,334,000
Other Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	75,439,000	146,509,000
Prior service cost (credit)	(265,575,000)	(303,417,000)
APS’s portion recorded as a regulatory (asset) liability	189,627,000	156,575,000
Income tax expense (benefit)	853,000	833,000
Accumulated other comprehensive loss	344,000	$ 500,000
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	0
Prior service credit	(37,842,000)
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2018	$ (37,842,000)