Other Income and Other Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other income:
Interest income	$ 1,957	$ 917	$ 6,256	$ 1,782
Debt return on Four Corners SCR deferral (Note 4)	4,910	0	11,190	0
Miscellaneous	91	174	95	273
Total other income	6,958	1,091	17,541	2,055
Other expense:
Non-operating costs	(2,480)	(1,978)	(7,404)	(7,338)
Investment losses — net	0	(231)	(268)	(759)
Miscellaneous	(2,583)	(2,784)	(4,391)	(4,398)
Total other expense	(5,063)	(4,993)	(12,063)	(12,495)
APS
Other income:
Interest income	1,151	683	4,874	1,278
Debt return on Four Corners SCR deferral (Note 4)	4,910	0	11,190	0
Miscellaneous	92	55	96	154
Total other income	6,153	738	16,160	1,432
Other expense:
Non-operating costs	(2,334)	(1,734)	(6,931)	(6,625)
Miscellaneous	(1,027)	(444)	(2,748)	(1,983)
Total other expense	$ (3,361)	$ (2,178)	$ (9,679)	$ (8,608)