Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,752,793	$ 5,553,188
OCI (loss) before reclassifications	262	292
Amounts reclassified from accumulated other comprehensive loss	1,022	1,225
Balance at end of period	5,806,680	5,596,832
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(60,725)	(56,522)
OCI (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	1,022	1,205
Balance at end of period	(59,703)	(55,317)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,071)	(574)
OCI (loss) before reclassifications	262	292
Amounts reclassified from accumulated other comprehensive loss	0	20
Balance at end of period	(1,809)	(262)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(62,796)	(57,096)
Balance at end of period	(61,512)	(55,579)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,345,185	5,998,803
OCI (loss) before reclassifications		292
Amounts reclassified from accumulated other comprehensive loss	927	1,033
Balance at end of period	6,386,275	6,040,344
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(40,918)	(34,948)
OCI (loss) before reclassifications		0
Amounts reclassified from accumulated other comprehensive loss	927	1,013
Balance at end of period	(39,991)	(33,935)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(574)
OCI (loss) before reclassifications		292
Amounts reclassified from accumulated other comprehensive loss	0	20
Balance at end of period	0	(262)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(40,918)	(35,522)
Balance at end of period	$ (39,991)	$ (34,197)