Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 57
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	217	(969)
Reclassification of postretirement liability adjustment from regulatory asset, tax	(725)	0
Foreign currency translation adjustments recognized during the period, tax	5	15
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(15)	5
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 19	$ 19