Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Jun. 30, 2018
Income Taxes
Reduction in net deferred income tax liabilities	$ 1,140,000
Regulatory liabilities, non-current	2,452,536	$ 2,389,002
APS
Income Taxes
Reduction in net deferred income tax liabilities	1,140,000
Regulatory liabilities, non-current	2,452,536	$ 2,389,002
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	$ 1,520,000