Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 5,553,188	$ 5,348,705
OCI (loss) before reclassifications	292
Amounts reclassified from accumulated other comprehensive loss	1,225	1,207
Balance at end of period	5,596,832	5,381,725
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(56,522)	(45,997)
OCI (loss) before reclassifications	0
Amounts reclassified from accumulated other comprehensive loss	1,205	879
Balance at end of period	(55,317)	(45,118)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(574)	(1,711)
OCI (loss) before reclassifications	292
Amounts reclassified from accumulated other comprehensive loss	20	328
Balance at end of period	(262)	(1,383)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(57,096)	(47,708)
Balance at end of period	(55,579)	(46,501)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	5,998,803	5,786,797
OCI (loss) before reclassifications	292
Amounts reclassified from accumulated other comprehensive loss	1,033	1,080
Balance at end of period	6,040,344	5,821,026
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(34,948)	(25,396)
OCI (loss) before reclassifications	0
Amounts reclassified from accumulated other comprehensive loss	1,013	752
Balance at end of period	(33,935)	(24,644)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(574)	(1,711)
OCI (loss) before reclassifications	292
Amounts reclassified from accumulated other comprehensive loss	20	328
Balance at end of period	(262)	(1,383)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(35,522)	(27,107)
Balance at end of period	$ (34,197)	$ (26,027)