CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Unaudited) - USD ($) $ in Thousands		Total	Common Stock	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests	APS	APS Common Stock	APS Additional Paid-In Capital	APS Retained Earnings	APS Accumulated Other Comprehensive Income (Loss)	APS Noncontrolling Interests
Beginning balance (in shares) at Dec. 31, 2016			111,392,053	55,317					71,264,947
Balance at beginning of period at Dec. 31, 2016		$ 4,935,912	$ 2,596,030	$ (4,133)	$ 2,255,547	$ (43,822)	$ 132,290	$ 5,037,970	$ 178,162	$ 2,421,696	$ 2,331,245	$ (25,423)	$ 132,290
Increase (Decrease) in Shareholders' Equity
Net income		200,502			190,755		9,747	202,017			192,270		9,747
Other comprehensive income		196				196		311				311
Dividends on common stock		(146,204)			(146,204)			(146,200)			(146,200)
Issuance of common stock (in shares)			250,627
Issuance of common stock		8,452	$ 8,452
Purchase of treasury stock (in shares)	[1]			(156,172)
Purchase of treasury stock	[1]	(12,430)		$ (12,430)
Reissuance of treasury stock for stock-based compensation and other (in shares)				192,191
Reissuance of treasury stock for stock-based compensation and other		15,021		$ 15,010	11		0
Net capital activities by noncontrolling interests		(11,372)					(11,372)						(11,372)
Ending balance (in shares) at Jun. 30, 2017			111,642,680	19,298					71,264,947
Balance at end of period at Jun. 30, 2017		4,990,077	$ 2,604,482	$ (1,553)	2,300,109	(43,626)	130,665	5,082,726	$ 178,162	2,421,696	2,377,315	(25,112)	130,665
Balance at beginning of period at Mar. 31, 2017		(42,863)										(24,375)
Increase (Decrease) in Shareholders' Equity
Net income		172,317						173,982
Other comprehensive income		(763)						(737)
Ending balance (in shares) at Jun. 30, 2017			111,642,680	19,298					71,264,947
Balance at end of period at Jun. 30, 2017		$ 4,990,077	$ 2,604,482	$ (1,553)	2,300,109	(43,626)	130,665	5,082,726	$ 178,162	2,421,696	2,377,315	(25,112)	130,665
Beginning balance (in shares) at Dec. 31, 2017		111,816,170	111,816,170	64,463					71,264,947
Balance at beginning of period at Dec. 31, 2017		$ 5,135,730	$ 2,614,805	$ (5,624)	2,442,511	(45,002)	129,040	5,385,869	$ 178,162	2,571,696	2,533,954	(26,983)	129,040
Increase (Decrease) in Shareholders' Equity
Net income		179,706			169,959		9,747	197,170			187,423		9,747
Other comprehensive income		(3,070)				(3,070)		(3,138)				(3,138)
Dividends on common stock		(155,620)			(155,620)			(155,599)			(155,599)
Issuance of common stock (in shares)			174,052
Issuance of common stock		9,867	$ 9,867
Purchase of treasury stock (in shares)	[1]			(81,177)
Purchase of treasury stock	[1]	(6,277)		$ (6,277)
Reissuance of treasury stock for stock-based compensation and other (in shares)				128,007
Reissuance of treasury stock for stock-based compensation and other		10,470		$ 10,470	0		0
Reclassification of income tax effects related to new tax reform (See Note 13)		(8,552)			8,552	(8,552)		(5,038)			5,038	(5,038)
Net capital activities by noncontrolling interests		$ (11,372)					(11,372)	(11,372)					(11,372)
Ending balance (in shares) at Jun. 30, 2018		111,990,222	111,990,222	17,633					71,264,947.000
Balance at end of period at Jun. 30, 2018		$ 5,159,434	$ 2,624,672	$ (1,431)	2,465,402	(56,624)	127,415	5,412,930	$ 178,162	2,571,696	2,570,816	(35,159)	127,415
Balance at beginning of period at Mar. 31, 2018						(52,341)						(30,851)
Increase (Decrease) in Shareholders' Equity
Net income		171,612						182,699
Other comprehensive income		$ (4,283)						(4,308)
Ending balance (in shares) at Jun. 30, 2018		111,990,222	111,990,222	17,633					71,264,947.000
Balance at end of period at Jun. 30, 2018		$ 5,159,434	$ 2,624,672	$ (1,431)	$ 2,465,402	$ (56,624)	$ 127,415	$ 5,412,930	$ 178,162	$ 2,571,696	$ 2,570,816	$ (35,159)	$ 127,415

[1] Primarily represents shares of common stock withheld from certain stock awards for tax purposes.