Retirement Plans and Other Benefits - Impact to Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	$ 735,186	$ 794,292
Prior service credit	0	0
APS’s portion recorded as a regulatory (asset) liability	(660,223)	(733,351)
Income tax expense (benefit)	(18,546)	(15,083)
Accumulated other comprehensive loss	56,417	45,858
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	33,642
Prior service credit	0
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2020	33,642
Other Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	12,238	63,544
Prior service credit	(189,912)	(227,733)
APS’s portion recorded as a regulatory (asset) liability	177,209	163,767
Income tax expense (benefit)	570	561
Accumulated other comprehensive loss	105	$ 139
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	(37,575)
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2020	$ (37,575)