Consolidated Statements of Comprehensive Income - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification adjustment for loss on derivative instruments included in net income, tax	$ (140)	$ 429	$ 224
Postretirement liability gains (losses) arising during the period, tax	(2,012)	1,471	(1,162)
Amortization of postretirement liability losses included in net periodic benefit cost, tax	476	721	645
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(876)
Foreign currency translation adjustment recognized during the period, tax	0	(14)	(3)
Reclassification adjustment for foreign currency translation adjustment included in net income, tax	0	75	0
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	35	(38)	(75)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	$ 10	$ 35	$ 65