Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 414,707	$ 367,489	$ 378,131
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $15, $177 and $145 in 2023, 2022 and 2021, respectively	81	413	446
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $(201), $3,965 and $1,626 in 2023, 2022 and 2021, respectively	(646)	12,007	4,876
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $78, $597 and $615 in 2023, 2022 and 2021, respectively	242	1,819	1,870
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0, $(1,086) and $0 in 2023, 2022 and 2021, respectively	0	(3,265)	0
Postretirement liability adjustment	(404)	10,561	6,746
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $46, $(177) and $(67) in 2023, 2022 and 2021, respectively	173	(667)	(252)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $11, $31 and $36 in 2023, 2022 and 2021, respectively	43	114	134
Net unrealized gain (loss) on available-for-sale investments	216	(553)	(118)
Other comprehensive income (loss)	(107)	10,421	7,074
Comprehensive income attributable to common stockholders	$ 414,600	$ 377,910	$ 385,205