Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 281,108	$ 414,707	$ 367,489
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $0, $15 and $177 in 2024, 2023 and 2022, respectively	0	81	413
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $360, $(201) and $3,965 in 2024, 2023 and 2022, respectively	1,049	(646)	12,007
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $145, $78 and $597 in 2024, 2023 and 2022, respectively	432	242	1,819
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0, $0 and $(1,086) in 2024, 2023 and 2022, respectively	0	0	(3,265)
Postretirement liability adjustment	1,481	(404)	10,561
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 $23, $46 and $(177) in 2024, 2023 and 2022, respectively	85	173	(667)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $5, $11 and $31 in 2024, 2023 and 2022, respectively	20	43	114
Net unrealized gain (loss) on available-for-sale investments	105	216	(553)
Other comprehensive income (loss)	1,586	(107)	10,421
Comprehensive income attributable to common stockholders	$ 282,694	$ 414,600	$ 377,910