Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,610,477	$ 5,357,243	$ 5,553,188	$ 5,348,705
OCI (loss) before reclassifications	(659)		(3,924)	(2,422)
Amounts reclassified from accumulated other comprehensive loss	1,043	1,098	3,529	3,592
Balance at end of period	5,967,175	5,681,705	5,967,175	5,681,705
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(56,326)	(46,657)	(56,522)	(45,997)
OCI (loss) before reclassifications	0		(2,008)	(2,422)
Amounts reclassified from accumulated other comprehensive loss	1,043	880	3,247	2,642
Balance at end of period	(55,283)	(45,777)	(55,283)	(45,777)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(1,549)	(979)	(574)	(1,711)
OCI (loss) before reclassifications	(659)		(1,916)	0
Amounts reclassified from accumulated other comprehensive loss	0	218	282	950
Balance at end of period	(2,208)	(761)	(2,208)	(761)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(57,875)	(47,636)	(57,096)	(47,708)
Balance at end of period	(57,491)	(46,538)	(57,491)	(46,538)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,054,137	5,797,857	5,998,803	5,786,797
OCI (loss) before reclassifications			(1,659)	(2,414)
Amounts reclassified from accumulated other comprehensive loss	900	973	3,056	3,218
Balance at end of period	6,410,275	6,122,571	6,410,275	6,122,571
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(35,025)	(26,297)	(34,948)	(25,396)
OCI (loss) before reclassifications			(1,951)	(2,414)
Amounts reclassified from accumulated other comprehensive loss	900	755	2,774	2,268
Balance at end of period	(34,125)	(25,542)	(34,125)	(25,542)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	0	(979)	(574)	(1,711)
OCI (loss) before reclassifications			292	0
Amounts reclassified from accumulated other comprehensive loss	0	218	282	950
Balance at end of period	0	(761)	0	(761)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(35,025)	(27,276)	(35,522)	(27,107)
Balance at end of period	$ (34,125)	$ (26,303)	$ (34,125)	$ (26,303)