Changes in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period		$ (42,863)	$ 5,135,730	$ 4,935,912
OCI (loss) before reclassifications	$ (5,928)	(2,150)	(6,024)	(2,920)
Amounts reclassified from accumulated other comprehensive loss	1,645	1,387	2,954	3,116
Reclassification of income tax effect related to tax reform			(8,552)
Balance at end of period	5,159,434	4,990,077	5,159,434	4,990,077
Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(49,494)	(38,548)	(42,440)	(39,070)
OCI (loss) before reclassifications	(5,928)	(2,157)	(5,928)	(2,157)
Amounts reclassified from accumulated other comprehensive loss	1,189	823	2,089	1,345
Reclassification of income tax effect related to tax reform			(7,954)
Balance at end of period	(54,233)	(39,882)	(54,233)	(39,882)
Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,847)	(4,315)	(2,562)	(4,752)
OCI (loss) before reclassifications	0	7	(96)	(763)
Amounts reclassified from accumulated other comprehensive loss	456	564	865	1,771
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period	(2,391)	(3,744)	(2,391)	(3,744)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(52,341)		(45,002)	(43,822)
Reclassification of income tax effect related to tax reform			(8,552)
Balance at end of period	(56,624)	(43,626)	(56,624)	(43,626)
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	(5,790)	(2,114)	(5,886)	(2,884)
Amounts reclassified from accumulated other comprehensive loss	1,482	1,377	2,748	3,195
Reclassification of income tax effect related to tax reform			(5,038)
Balance at end of period	5,412,930	5,082,726	5,412,930	5,082,726
APS | Pension and Other Postretirement Benefits
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(28,004)	(20,060)	(24,421)	(20,671)
OCI (loss) before reclassifications	(5,790)	(2,121)	(5,790)	(2,121)
Amounts reclassified from accumulated other comprehensive loss	1,026	813	1,883	1,424
Reclassification of income tax effect related to tax reform			(4,440)
Balance at end of period	(32,768)	(21,368)	(32,768)	(21,368)
APS | Derivative Instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(2,847)	(4,315)	(2,562)	(4,752)
OCI (loss) before reclassifications	0	7	(96)	(763)
Amounts reclassified from accumulated other comprehensive loss	456	564	865	1,771
Reclassification of income tax effect related to tax reform			(598)
Balance at end of period	(2,391)	(3,744)	(2,391)	(3,744)
APS | Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(30,851)	(24,375)	(26,983)	(25,423)
Reclassification of income tax effect related to tax reform			(5,038)
Balance at end of period	$ (35,159)	$ (25,112)	$ (35,159)	$ (25,112)