Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 18,311	$ 64,615	$ 114,056	$ 225,950
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $33 and $33 for the three months ended and $85 and $101 for the nine months ended in 2025 and 2024, respectively	103	104	324	311
Net unrealized gain on available-for-sale investments:
Net unrealized (loss) gain on available-for-sale investments arising during the period, net of tax of $(1) and $47 for the three months ended and $22 and $45 for the nine months ended in 2025 and 2024, respectively	(3)	177	82	170
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $1 and $1 for the three months ended and $2 and $5 for the nine months ended in 2025 and 2024, respectively	3	3	9	16
Net unrealized gain on available-for-sale investments	0	180	91	186
Other comprehensive income	103	284	415	497
Comprehensive income attributable to common stockholders	$ 18,414	$ 64,899	$ 114,471	$ 226,447