Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification adjustment for (gain) loss on derivative instruments included in net income (loss), tax	$ 226	$ 233	$ 240
Reclassification adjustment for (gain) loss on derivative instruments included in income (loss) from discontinued operations, tax	0	0	173
Postretirement liability gains (losses) arising during the period, tax	(836)	(55)	(7,665)
Amortization of postretirement liability (gains) losses included in net periodic benefit cost (credit), tax	1,425	1,128	492
Reclassification of postretirement liability adjustment to regulatory asset, tax	0	1,416	4,509
Foreign currency translation adjustment recognized during the period, tax	31	(105)	(99)
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income (loss), tax	0	490	0
Net unrealized gains (losses) on available-for-sale investments arising during the period, tax	(98)	(91)	(83)
Reclassification adjustments for (gain) loss on available-for-sale investments included in net income (loss), tax	$ 77	$ 70	$ 73