Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,596,832	$ 5,381,725	$ 5,553,188	$ 5,348,705
OCI (loss) before reclassifications	(3,557)	(2,422)	(3,265)	(2,422)
Amounts reclassified from accumulated other comprehensive loss	1,261	1,287	2,486	2,494
Balance at end of period	5,610,477	5,357,243	5,610,477	5,357,243
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(55,317)	(45,118)	(56,522)	(45,997)
OCI (loss) before reclassifications	(2,008)	(2,422)	(2,008)	(2,422)
Amounts reclassified from accumulated other comprehensive loss	999	883	2,204	1,762
Balance at end of period	(56,326)	(46,657)	(56,326)	(46,657)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(262)	(1,383)	(574)	(1,711)
OCI (loss) before reclassifications	(1,549)	0	(1,257)	0
Amounts reclassified from accumulated other comprehensive loss	262	404	282	732
Balance at end of period	(1,549)	(979)	(1,549)	(979)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(55,579)	(46,501)	(57,096)	(47,708)
Balance at end of period	(57,875)	(47,636)	(57,875)	(47,636)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	6,040,344	5,821,026	5,998,803	5,786,797
OCI (loss) before reclassifications	(1,951)	(2,414)	(1,659)	(2,414)
Amounts reclassified from accumulated other comprehensive loss	1,123	1,165	2,156	2,245
Balance at end of period	6,054,137	5,797,857	6,054,137	5,797,857
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(33,935)	(24,644)	(34,948)	(25,396)
OCI (loss) before reclassifications	(1,951)	(2,414)	(1,951)	(2,414)
Amounts reclassified from accumulated other comprehensive loss	861	761	1,874	1,513
Balance at end of period	(35,025)	(26,297)	(35,025)	(26,297)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(262)	(1,383)	(574)	(1,711)
OCI (loss) before reclassifications	0	0	292	0
Amounts reclassified from accumulated other comprehensive loss	262	404	282	732
Balance at end of period	0	(979)	0	(979)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(34,197)	(26,027)	(35,522)	(27,107)
Balance at end of period	$ (35,025)	$ (27,276)	$ (35,025)	$ (27,276)