Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,806,680	$ 5,596,832	$ 5,752,793	$ 5,553,188
OCI (loss) before reclassifications	(255)	(3,557)	7	(3,265)
Amounts reclassified from accumulated other comprehensive loss	1,189	1,261	2,211	2,486
Balance at end of period	5,834,899	5,610,477	5,834,899	5,610,477
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(59,703)	(55,317)	(60,725)	(56,522)
OCI (loss) before reclassifications	(1,125)	(2,008)	(1,125)	(2,008)
Amounts reclassified from accumulated other comprehensive loss	1,189	999	2,211	2,204
Balance at end of period	(59,639)	(56,326)	(59,639)	(56,326)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(1,809)	(262)	(2,071)	(574)
OCI (loss) before reclassifications	870	(1,549)	1,132	(1,257)
Amounts reclassified from accumulated other comprehensive loss	0	262	0	282
Balance at end of period	(939)	(1,549)	(939)	(1,549)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(61,512)	(55,579)	(62,796)	(57,096)
Balance at end of period	(60,578)	(57,875)	(60,578)	(57,875)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,386,275	6,040,344	6,345,185	5,998,803
OCI (loss) before reclassifications	(914)	(1,951)	(914)	(1,659)
Amounts reclassified from accumulated other comprehensive loss	1,073	1,123	2,000	2,156
Balance at end of period	6,412,290	6,054,137	6,412,290	6,054,137
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(39,991)	(33,935)	(40,918)	(34,948)
OCI (loss) before reclassifications	(914)	(1,951)	(914)	(1,951)
Amounts reclassified from accumulated other comprehensive loss	1,073	861	2,000	1,874
Balance at end of period	(39,832)	(35,025)	(39,832)	(35,025)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(262)	0	(574)
OCI (loss) before reclassifications	0	0	0	292
Amounts reclassified from accumulated other comprehensive loss	0	262	0	282
Balance at end of period	0	0	0	0
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(39,991)	(34,197)	(40,918)	(35,522)
Balance at end of period	$ (39,832)	$ (35,025)	$ (39,832)	$ (35,025)