Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 13,780	$ 60,436	$ 95,745	$ 161,334
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $33 and $34 for the three months ended and $51 and $68 for the six months ended in 2025 and 2024, respectively	103	103	221	207
Net unrealized gain on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $4 and $8 for the three months ended and $23 and $(2) for the six months ended in 2025 and 2024, respectively	14	29	85	(7)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $1 and $2 for the three months ended and $1 and $3 for the six months ended in 2025 and 2024, respectively	3	7	6	13
Net unrealized gain on available-for-sale investments	17	36	91	6
Other comprehensive income	120	139	312	213
Comprehensive income attributable to common stockholders	$ 13,900	$ 60,575	$ 96,057	$ 161,547