Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 64,615	$ 74,929	$ 225,950	$ 243,974
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $17 for the nine months ended 2023	0	0	0	81
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $33 and $98 for the three months ended and $101 and $263 for the nine months ended in 2024 and 2023, respectively	104	(6)	311	481
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 $47 and $(17) for the three months ended and $45 and $(21) for the nine months ended in 2024 and 2023, respectively	177	(66)	170	(80)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $1 and $3 for the three months ended and $5 and $9 for the nine months ended in 2024 and 2023, respectively	3	10	16	36
Net unrealized gain (loss) on available-for-sale investments	180	(56)	186	(44)
Other comprehensive income (loss)	284	(62)	497	518
Comprehensive income attributable to common stockholders	$ 64,899	$ 74,867	$ 226,447	$ 244,492