Income Taxes - Components of Deferred Income Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
DEFERRED TAX ASSETS
Risk management activities	$ 17,552		$ 15,785
Regulatory liabilities:
Excess deferred income taxes - Tax Cuts and Jobs Act	335,877		376,869
Asset retirement obligation and removal costs	143,011		117,201
Unamortized investment tax credits	52,236		53,284
Other postretirement liabilities	43,841		40,532
Other	52,382		40,380
Pension liabilities	73,210		112,019
Coal reclamation liabilities	40,837		47,508
Renewable energy incentives	28,066		30,779
Credit and loss carryforwards	54,795		1,755
Other	63,102		58,820
Total deferred tax assets	904,909		894,932
DEFERRED TAX LIABILITIES
Plant-related	(2,448,458)		(2,277,724)
Risk management activities	(27)		(237)
Other postretirement assets and other special use funds	(66,399)		(57,697)
Regulatory assets:
Allowance for equity funds used during construction	(40,023)		(39,086)
Deferred fuel and purchased power	(35,162)		(23,086)
Pension benefits	(163,339)		(181,504)
Retired power plant costs (see Note 4)	(42,228)		(48,348)
Other	(82,722)		(72,096)
Other	(18,890)		(2,575)
Total deferred tax liabilities	(2,897,248)	$ (56,000)	(2,702,353)
Deferred income taxes — net	(1,992,339)		(1,807,421)
ARIZONA PUBLIC SERVICE COMPANY
DEFERRED TAX ASSETS
Risk management activities	17,552		15,785
Regulatory liabilities:
Excess deferred income taxes - Tax Cuts and Jobs Act	335,877		376,869
Asset retirement obligation and removal costs	143,011		117,201
Unamortized investment tax credits	52,236		53,284
Other postretirement liabilities	43,841		40,532
Other	52,382		40,380
Pension liabilities	67,976		107,009
Coal reclamation liabilities	40,837		47,508
Renewable energy incentives	28,066		30,779
Credit and loss carryforwards	10,992		0
Other	70,948		59,919
Total deferred tax assets	863,718		889,266
DEFERRED TAX LIABILITIES
Plant-related	(2,448,458)		(2,277,724)
Risk management activities	(27)		(237)
Other postretirement assets and other special use funds	(65,965)		(57,274)
Regulatory assets:
Allowance for equity funds used during construction	(40,023)		(39,086)
Deferred fuel and purchased power	(35,162)		(23,086)
Pension benefits	(163,339)		(181,504)
Retired power plant costs (see Note 4)	(42,228)		(48,348)
Other	(82,722)		(72,096)
Other	(18,890)		(2,575)
Total deferred tax liabilities	(2,896,814)		(2,701,930)
Deferred income taxes — net	$ (2,033,096)		$ (1,812,664)