Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,834,899	$ 5,610,477	$ 5,752,793	$ 5,553,188
OCI (loss) before reclassifications	(194)	(659)	(187)	(3,924)
Amounts reclassified from accumulated other comprehensive loss	1,106	1,043	3,317	3,529
Balance at end of period	6,186,464	5,967,175	6,186,464	5,967,175
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,412,290	6,054,137	6,345,185	5,998,803
OCI (loss) before reclassifications	0	0	(914)	(1,659)
Amounts reclassified from accumulated other comprehensive loss	1,000	900	3,000	3,056
Balance at end of period	6,761,259	6,410,275	6,761,259	6,410,275
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(59,639)	(56,326)	(60,725)	(56,522)
OCI (loss) before reclassifications	0	0	(1,125)	(2,008)
Amounts reclassified from accumulated other comprehensive loss	1,106	1,043	3,317	3,247
Balance at end of period	(58,533)	(55,283)	(58,533)	(55,283)
Pension and Other Postretirement Benefits | APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(39,832)	(35,025)	(40,918)	(34,948)
OCI (loss) before reclassifications	0	0	(914)	(1,951)
Amounts reclassified from accumulated other comprehensive loss	1,000	900	3,000	2,774
Balance at end of period	(38,832)	(34,125)	(38,832)	(34,125)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(939)	(1,549)	(2,071)	(574)
OCI (loss) before reclassifications	(194)	(659)	938	(1,916)
Amounts reclassified from accumulated other comprehensive loss	0	0	0	282
Balance at end of period	(1,133)	(2,208)	(1,133)	(2,208)
Derivative Instruments | APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	0	0	(574)
OCI (loss) before reclassifications	0	0	0	292
Amounts reclassified from accumulated other comprehensive loss	0	0	0	282
Balance at end of period	0	0	0	0
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(60,578)	(57,875)	(62,796)	(57,096)
Balance at end of period	(59,666)	(57,491)	(59,666)	(57,491)
Accumulated Other Comprehensive Income (Loss) | APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(39,832)	(35,025)	(40,918)	(35,522)
Balance at end of period	$ (38,832)	$ (34,125)	$ (38,832)	$ (34,125)