Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 60,436	$ 130,692	$ 161,334	$ 169,044
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $6 and $17 for the three and six months ended 2023, respectively	0	47	0	81
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $34 and $131 for the three months ended and $68 and $165 for the six months ended in 2024 and 2023, respectively	103	387	207	487
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 $8 and $(22) for the three months ended and $(2) and $(4) for the six months ended in 2024 and 2023, respectively	29	(84)	(7)	(14)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $2 and $3 for the three months ended and $3 and $6 for the six months ended in 2024 and 2023, respectively	7	13	13	26
Net unrealized gain (loss) on available-for-sale investments	36	(71)	6	12
Other comprehensive income	139	363	213	580
Comprehensive income attributable to common stockholders	$ 60,575	$ 131,055	$ 161,547	$ 169,624