Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net unrealized gain (loss) on derivative instruments arising during the period, tax	$ 0	$ 0	$ (3,116)
Reclassification adjustment for (gain) loss on derivative instruments included in net income, tax	233	240	339
Reclassification adjustment for (gain) loss on derivative instruments included in discontinued operations, tax	0	173	(2,887)
Postretirement liability gains (losses) arising during the period, tax	(55)	(7,665)	11,818
Amortization of postretirement liability losses included in net periodic benefit cost (credit), tax	1,128	492	1,276
Reclassification of postretirement liability adjustment to regulatory asset, tax	1,416	4,509	0
Foreign currency translation adjustment recognized during the period, tax	(105)	(99)	(177)
Reclassification adjustment for (gain) loss on foreign currency translation adjustment included in net income, tax	490	0	70
Net unrealized gains (losses) on available-for-sale investments arising during the period, tax	(91)	(83)	(105)
Reclassification adjustments for (gain) loss on available-for-sale investments included in net income, tax	$ 70	$ 73	$ 59