Retirement Plans and Other Benefits - Impact to Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Details related to accumulated other comprehensive loss
Accumulated other comprehensive loss	$ 37,593	$ 57,756
Other Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	127,124	148,006
Prior service cost (credit)	(341,301)	(379,269)
APS's portion recorded as a regulatory asset	213,621	230,916
Income tax benefit	925	851
Accumulated other comprehensive loss	369	504
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	3,784
Prior service cost (credit)	(37,884)
Total amounts estimated to be amortized from accumulated other comprehensive loss and regulatory assets in 2014	(34,100)
Pension Benefits
Details related to accumulated other comprehensive loss
Net actuarial loss	679,501	577,976
Prior service cost (credit)	609	1,203
APS's portion recorded as a regulatory asset	(619,223)	(485,037)
Income tax benefit	(23,663)	(36,890)
Accumulated other comprehensive loss	37,224	$ 57,252
Estimated amounts that will be amortized from accumulated other comprehensive loss and regulatory assets into net periodic benefit cost in 2014
Net actuarial loss	38,923
Prior service cost (credit)	527
Total amounts estimated to be amortized from accumulated other comprehensive loss and regulatory assets in 2014	$ 39,450