Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ 5,348,705	$ 5,135,730
OCI (loss) before reclassifications			(96)
Amounts reclassified from accumulated other comprehensive loss		1,207	1,309
Reclassification of income tax effect related to tax reform			(8,552)
Balance at end of period		5,381,725	5,153,671
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(45,997)	(42,440)
OCI (loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive loss		879	900
Reclassification of income tax effect related to tax reform			(7,954)
Balance at end of period		(45,118)	(49,494)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(1,711)	(2,562)
OCI (loss) before reclassifications			(96)
Amounts reclassified from accumulated other comprehensive loss		328	409
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period		(1,383)	(2,847)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(47,708)	(45,002)
Reclassification of income tax effect related to tax reform	[1]		(8,552)
Balance at end of period		(46,501)	(52,341)
APS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		5,786,797	5,385,869
OCI (loss) before reclassifications			(96)
Amounts reclassified from accumulated other comprehensive loss		1,080	1,266
Reclassification of income tax effect related to tax reform			(5,038)
Balance at end of period		5,821,026	5,401,512
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(25,396)	(24,421)
OCI (loss) before reclassifications			0
Amounts reclassified from accumulated other comprehensive loss		752	857
Reclassification of income tax effect related to tax reform			(4,440)
Balance at end of period		(24,644)	(28,004)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(1,711)	(2,562)
OCI (loss) before reclassifications			(96)
Amounts reclassified from accumulated other comprehensive loss		328	409
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period		(1,383)	(2,847)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		(27,107)	(26,983)
Reclassification of income tax effect related to tax reform	[1]		(5,038)
Balance at end of period		$ (26,027)	$ (30,851)

[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.