Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ 5,357,243	$ 5,159,434	$ 5,348,705	$ 5,135,730
OCI (loss) before reclassifications				(2,422)	(6,024)
Amounts reclassified from accumulated other comprehensive loss		1,098	1,550	3,592	4,504
Reclassification of income tax effect related to tax reform					(8,552)
Balance at end of period		5,681,705	5,485,861	5,681,705	5,485,861
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(46,657)	(54,233)	(45,997)	(42,440)
OCI (loss) before reclassifications				(2,422)	(5,928)
Amounts reclassified from accumulated other comprehensive loss		880	1,099	2,642	3,188
Reclassification of income tax effect related to tax reform					(7,954)
Balance at end of period		(45,777)	(53,134)	(45,777)	(53,134)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(979)	(2,391)	(1,711)	(2,562)
OCI (loss) before reclassifications				0	(96)
Amounts reclassified from accumulated other comprehensive loss		218	451	950	1,316
Reclassification of income tax effect related to tax reform					(598)
Balance at end of period		(761)	(1,940)	(761)	(1,940)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(47,636)	(56,624)	(47,708)	(45,002)
Reclassification of income tax effect related to tax reform	[1]				(8,552)
Balance at end of period		(46,538)	(55,074)	(46,538)	(55,074)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		5,797,857	5,412,930	5,786,797	5,385,869
Amounts reclassified from accumulated other comprehensive loss		973	1,403
Balance at end of period		6,122,571	5,757,571	6,122,571	5,757,571
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(26,297)	(32,768)	(25,396)	(24,421)
OCI (loss) before reclassifications				(2,414)	(5,791)
Amounts reclassified from accumulated other comprehensive loss		755	952	2,268	2,836
Reclassification of income tax effect related to tax reform					(4,440)
Balance at end of period		(25,542)	(31,816)	(25,542)	(31,816)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(979)	(2,391)	(1,711)	(2,562)
OCI (loss) before reclassifications				0	(96)
Amounts reclassified from accumulated other comprehensive loss		218	451	950	1,316
Reclassification of income tax effect related to tax reform					(598)
Balance at end of period		(761)	(1,940)	(761)	(1,940)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(27,276)	(35,159)	(27,107)	(26,983)
OCI (loss) before reclassifications				(2,414)	(5,887)
Amounts reclassified from accumulated other comprehensive loss				3,218	4,152
Reclassification of income tax effect related to tax reform	[2]				(5,038)
Balance at end of period		$ (26,303)	$ (33,756)	$ (26,303)	$ (33,756)

[1] In 2018, the Company adopted new accounting guidance and elected to reclassify income tax effects of the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) on items within accumulated other comprehensive income to retained earnings.

[2] In 2018, the Company adopted new accounting guidance and elected to reclassify income tax effects of the Tax Act on items within accumulated other comprehensive income to retained earnings.