Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ (43,626)	$ 5,135,730	$ 4,935,912
OCI (loss) before reclassifications		9	(6,024)	(2,911)
Amounts reclassified from accumulated other comprehensive loss	$ 1,550	1,500	4,504	4,616
Reclassification of income tax effect related to tax reform			(8,552)
Balance at end of period	5,485,861	5,277,607	5,485,861	5,277,607
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(54,233)	(39,881)	(42,440)	(39,070)
OCI (loss) before reclassifications		0	(5,928)	(2,157)
Amounts reclassified from accumulated other comprehensive loss	1,099	790	3,188	2,136
Reclassification of income tax effect related to tax reform			(7,954)
Balance at end of period	(53,134)	(39,091)	(53,134)	(39,091)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,391)	(3,745)	(2,562)	(4,752)
OCI (loss) before reclassifications		9	(96)	(754)
Amounts reclassified from accumulated other comprehensive loss	451	710	1,316	2,480
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period	(1,940)	(3,026)	(1,940)	(3,026)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(56,624)		(45,002)	(43,822)
Reclassification of income tax effect related to tax reform			(8,552)
Balance at end of period	(55,074)	(42,117)	(55,074)	(42,117)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period			5,385,869	5,037,970
OCI (loss) before reclassifications		9	(5,887)	(2,875)
Amounts reclassified from accumulated other comprehensive loss	1,403	1,487	4,152	4,682
Reclassification of income tax effect related to tax reform			(5,038)
Balance at end of period	5,757,571	5,373,351	5,757,571	5,373,351
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(32,768)	(21,367)	(24,421)	(20,671)
OCI (loss) before reclassifications		0	(5,791)	(2,121)
Amounts reclassified from accumulated other comprehensive loss	952	777	2,836	2,202
Reclassification of income tax effect related to tax reform			(4,440)
Balance at end of period	(31,816)	(20,590)	(31,816)	(20,590)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,391)	(3,745)	(2,562)	(4,752)
OCI (loss) before reclassifications		9	(96)	(754)
Amounts reclassified from accumulated other comprehensive loss	451	710	1,316	2,480
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period	(1,940)	(3,026)	(1,940)	(3,026)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(35,159)	(25,112)	(26,983)	(25,423)
Reclassification of income tax effect related to tax reform			(5,038)
Balance at end of period	$ (33,756)	$ (23,616)	$ (33,756)	$ (23,616)