CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
NET INCOME	$ 171,612	$ 172,317	$ 179,706	$ 200,502
Derivative instruments:
Net unrealized gain (loss), net of tax expense	0	7	(96)	(763)
Reclassification of net realized loss, net of tax expense	456	564	865	1,771
Pension and other postretirement benefits activity, net of tax benefit	(4,739)	(1,334)	(3,839)	(812)
Total other comprehensive income	(4,283)	(763)	(3,070)	196
COMPREHENSIVE INCOME	167,329	171,554	176,636	200,698
Less: Comprehensive income attributable to noncontrolling interests	4,874	4,874	9,747	9,747
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	162,455	166,680	166,889	190,951
APS
NET INCOME	182,699	173,982	197,170	202,017
Derivative instruments:
Net unrealized gain (loss), net of tax expense	0	7	(96)	(763)
Reclassification of net realized loss, net of tax expense	456	564	865	1,771
Pension and other postretirement benefits activity, net of tax benefit	(4,764)	(1,308)	(3,907)	(697)
Total other comprehensive income	(4,308)	(737)	(3,138)	311
COMPREHENSIVE INCOME	178,391	173,245	194,032	202,328
Less: Comprehensive income attributable to noncontrolling interests	4,874	4,874	9,747	9,747
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	$ 173,517	$ 168,371	$ 184,285	$ 192,581