Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 56	$ 56	$ 112	$ 114
Amortization of postretirement liability (gains) losses included in net periodic benefit cost, tax	190	150	406	(819)
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(725)	0
Foreign currency translation adjustments recognized during the period, tax	(9)	19	(3)	33
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(13)	(16)	(28)	(10)
Reclassification adjustment for loss (gain) on available-for-sale investments included in net income (loss), tax	$ 17	$ 19	$ 36	$ 37