Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 56	$ 168	$ 170
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	203	143	609	(676)
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(725)	0
Foreign currency translation adjustments recognized during the period, tax	9	(2)	5	32
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(10)	(23)	(38)	(35)
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 14	$ 18	$ 50	$ 57