Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ 5,381,725	$ 5,153,671	$ 5,348,705	$ 5,135,730
OCI (loss) before reclassifications		(2,422)	(5,928)	(2,422)	(6,024)
Amounts reclassified from accumulated other comprehensive loss		1,287	1,645	2,494	2,954
Reclassification of income tax effect related to tax reform					(8,552)
Balance at end of period		5,357,243	5,159,434	5,357,243	5,159,434
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(45,118)	(49,494)	(45,997)	(42,440)
OCI (loss) before reclassifications		(2,422)	(5,928)	(2,422)	(5,928)
Amounts reclassified from accumulated other comprehensive loss		883	1,189	1,762	2,089
Reclassification of income tax effect related to tax reform					(7,954)
Balance at end of period		(46,657)	(54,233)	(46,657)	(54,233)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(1,383)	(2,847)	(1,711)	(2,562)
OCI (loss) before reclassifications		0	0	0	(96)
Amounts reclassified from accumulated other comprehensive loss		404	456	732	865
Reclassification of income tax effect related to tax reform					(598)
Balance at end of period		(979)	(2,391)	(979)	(2,391)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(46,501)	(52,341)	(47,708)	(45,002)
Reclassification of income tax effect related to tax reform	[1]				(8,552)
Balance at end of period		(47,636)	(56,624)	(47,636)	(56,624)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		5,821,026	5,401,512	5,786,797	5,385,869
OCI (loss) before reclassifications		(2,414)	(5,790)
Amounts reclassified from accumulated other comprehensive loss		1,165	1,482
Balance at end of period		5,797,857	5,412,930	5,797,857	5,412,930
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(24,644)	(28,004)	(25,396)	(24,421)
OCI (loss) before reclassifications		(2,414)	(5,790)	(2,414)	(5,790)
Amounts reclassified from accumulated other comprehensive loss		761	1,026	1,513	1,883
Reclassification of income tax effect related to tax reform					(4,440)
Balance at end of period		(26,297)	(32,768)	(26,297)	(32,768)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(1,383)	(2,847)	(1,711)	(2,562)
OCI (loss) before reclassifications		0	0	0	(96)
Amounts reclassified from accumulated other comprehensive loss		404	456	732	865
Reclassification of income tax effect related to tax reform					(598)
Balance at end of period		(979)	(2,391)	(979)	(2,391)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(26,027)	(30,851)	(27,107)	(26,983)
OCI (loss) before reclassifications				(2,414)	(5,886)
Amounts reclassified from accumulated other comprehensive loss				2,245	2,748
Reclassification of income tax effect related to tax reform	[2]				(5,038)
Balance at end of period		$ (27,276)	$ (35,159)	$ (27,276)	$ (35,159)

[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.