Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification adjustment for loss on derivative instruments included in net income, tax	$ 53	$ 56	$ 109	$ 112
Amortization of postretirement liability losses included in net periodic benefit cost (credit), tax	145	190	300	406
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	0	(725)
Foreign currency translation adjustments recognized during the period, tax	(13)	(9)	(14)	(3)
Reclassification of foreign currency translation adjustment	75	0	75	0
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(12)	(13)	(39)	(28)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	$ 10	$ 17	$ 17	$ 36