Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	$ 6,827,940	$ 6,316,322	$ 6,857,478	$ 6,284,862
Ending balance	7,211,093	6,726,070	7,211,093	6,726,070
Accumulated Other Comprehensive Loss
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(30,441)	(33,294)	(30,942)	(33,144)
Other comprehensive loss before reclassifications	(128)	(33)	(575)	(1,211)
Amounts reclassified from accumulated other comprehensive loss	478	513	1,426	1,541
Ending balance	(30,091)	(32,814)	(30,091)	(32,814)
Pension and Other Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(31,216)	(34,505)	(31,661)	(34,754)
Other comprehensive loss before reclassifications	0	0	(503)	(779)
Amounts reclassified from accumulated other comprehensive loss	478	513	1,426	1,541
Ending balance	(30,738)	(33,992)	(30,738)	(33,992)
Derivative Instruments
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	775	1,211	719	1,610
Other comprehensive loss before reclassifications	(128)	(33)	(72)	(432)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Ending balance	647	1,178	647	1,178
APS
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	8,665,366	7,824,320	8,376,332	7,349,136
Ending balance	9,035,106	8,236,800	9,035,106	8,236,800
APS | Accumulated Other Comprehensive Loss
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(13,846)	(17,036)	(14,116)	(17,219)
Ending balance	(13,459)	(16,586)	(13,459)	(16,586)
APS | Pension and Other Postretirement Benefits
Changes in accumulated other comprehensive income (loss) by component
Beginning balance	(13,846)	(17,036)	(14,116)	(17,219)
Other comprehensive loss before reclassifications	0	0	(504)	(718)
Amounts reclassified from accumulated other comprehensive loss	387	450	1,161	1,351
Ending balance	$ (13,459)	$ (16,586)	$ (13,459)	$ (16,586)