Other Income and Other Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other income:
Interest income	$ 2,408	$ 387	$ 4,299	$ 864
Debt return on Four Corners SCR deferral (Note 4)	4,188	0	6,280	0
Miscellaneous	2	97	4	100
Total other income	6,598	484	10,583	964
Other expense:
Non-operating costs	(3,278)	(3,401)	(4,924)	(5,360)
Investment losses — net	(174)	(227)	(350)	(528)
Miscellaneous	(319)	(194)	(1,726)	(1,614)
Total other expense	(3,771)	(3,822)	(7,000)	(7,502)
APS
Other income:
Interest income	2,046	257	3,724	596
Debt return on Four Corners SCR deferral (Note 4)	4,188	0	6,280	0
Miscellaneous	1	95	3	98
Total other income	6,235	352	10,007	694
Other expense:
Non-operating costs	(3,057)	(3,149)	(4,596)	(4,899)
Miscellaneous	(315)	(152)	(1,722)	(1,530)
Total other expense	$ (3,372)	$ (3,301)	$ (6,318)	$ (6,429)