Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 56
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	155	217
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	(725)
Foreign currency translation adjustments recognized during the period, tax	(1)	5
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(28)	(15)
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 7	$ 19