Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Reclassification adjustment for loss on derivative instruments included in net income, tax	$ 15	$ 177	$ 145
Postretirement liability gains (losses) arising during the period, tax	(201)	3,965	1,626
Amortization of postretirement liability losses included in net periodic benefit credit, tax	78	597	615
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	(1,086)	0
Net unrealized loss on available-for-sale investments arising during the period, tax	46	(177)	(67)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	$ 11	$ 31	$ 36