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		481,447			466,827		14,620	491,143			476,523		14,620
Other comprehensive income		1,705				1,705		1,807				1,807
Dividends on common stock ($1.31 per share)		(146,204)			(146,204)			(146,198)			(146,198)
Issuance of common stock (in shares)			274,823
Issuance of common stock		12,795	$ 12,795
Purchase of treasury stock (in shares)	[1]			(162,312)
Purchase of treasury stock	[1]	(12,964)		$ (12,964)
Reissuance of treasury stock for stock-based compensation and other (in shares)				207,765
Reissuance of treasury stock for stock-based compensation and other		16,288		$ 16,264	23		1
Net capital activities by noncontrolling interests		(11,372)					(11,372)	(11,372)					(11,372)
Ending balance (in shares) at Sep. 30, 2017			111,666,876	9,864					71,264,947
Increase (Decrease) in Shareholders' Equity
Other								1			0		1
Balance at end of period at Sep. 30, 2017		5,277,607	$ 2,608,825	$ (833)	2,576,193	(42,117)	135,539	5,373,351	$ 178,162	2,421,696	2,661,570	(23,616)	135,539
Balance at beginning of period at Jun. 30, 2017		(43,626)										(25,112)
Increase (Decrease) in Shareholders' Equity
Net income		280,945						289,129
Other comprehensive income		1,509						1,496
Ending balance (in shares) at Sep. 30, 2017			111,666,876	9,864					71,264,947
Balance at end of period at Sep. 30, 2017		$ 5,277,607	$ 2,608,825	$ (833)	2,576,193	(42,117)	135,539	5,373,351	$ 178,162	2,421,696	2,661,570	(23,616)	135,539
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		499,591			484,971		14,620	540,409			525,789		14,620
Other comprehensive income		(1,520)				(1,520)		(1,735)				(1,735)
Dividends on common stock ($1.31 per share)		(155,607)			(155,607)			(155,601)			(155,601)
Issuance of common stock (in shares)			199,779
Issuance of common stock		14,822	$ 14,822
Purchase of treasury stock (in shares)	[1]			(81,278)
Purchase of treasury stock	[1]	(6,285)		$ (6,285)
Reissuance of treasury stock for stock-based compensation and other (in shares)				128,373
Reissuance of treasury stock for stock-based compensation and other		10,501		$ 10,500	1		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 Sep. 30, 2018		112,015,949	112,015,949	17,368					71,264,947
Increase (Decrease) in Shareholders' Equity
Other		$ 1						1					1
Balance at end of period at Sep. 30, 2018		5,485,861	$ 2,629,627	$ (1,409)	2,780,428	(55,074)	132,289	5,757,571	$ 178,162	2,571,696	2,909,180	(33,756)	132,289
Balance at beginning of period at Jun. 30, 2018						(56,624)						(35,159)
Increase (Decrease) in Shareholders' Equity
Net income		319,885						343,239
Other comprehensive income		$ 1,550						1,403
Ending balance (in shares) at Sep. 30, 2018		112,015,949	112,015,949	17,368					71,264,947
Balance at end of period at Sep. 30, 2018		$ 5,485,861	$ 2,629,627	$ (1,409)	$ 2,780,428	$ (55,074)	$ 132,289	$ 5,757,571	$ 178,162	$ 2,571,696	$ 2,909,180	$ (33,756)	$ 132,289

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