Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Income Taxes
Reduction in net deferred income tax liabilities	$ 11,000	$ 1,140,000
Regulatory liabilities, non-current	2,410,597	2,452,536
APS
Income Taxes
Reduction in net deferred income tax liabilities		1,140,000
Regulatory liabilities, non-current	2,410,597	2,452,536
Gross-up for revenue requirement of rate regulation		377,000
Excess deferred income taxes - ACC - Tax Cuts and Jobs Act | APS
Income Taxes
Regulatory liabilities, non-current	$ 9,000	$ 1,520,000