Retirement Plans and Other Benefits - Impact to Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Details related to accumulated other comprehensive loss
Accumulated other comprehensive loss	$ 39,070	$ 37,593
Other Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	146,509	127,124
Prior service cost (credit)	(303,417)	(341,301)
APS’s portion recorded as a regulatory (asset) liability	156,575	213,621
Income tax expense (benefit)	833	925
Accumulated other comprehensive loss	500	369
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	5,181
Prior service cost (credit)	(37,842)
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2017	(32,661)
Pension Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	773,750	679,501
Prior service cost (credit)	81	609
APS’s portion recorded as a regulatory (asset) liability	(711,059)	(619,223)
Income tax expense (benefit)	(24,202)	(23,663)
Accumulated other comprehensive loss	38,570	$ 37,224
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	46,971
Prior service cost (credit)	81
Total amounts estimated to be amortized from accumulated other comprehensive loss (gain) and regulatory assets (liabilities) in 2017	$ 47,052