Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 100,898	$ 38,353
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $11 for the three months ended 2023	0	34
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $34 and $34 for the three months ended in 2024 and 2023, respectively	104	100
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized (loss) gain on available-for-sale investments arising during the period, net of tax of $(10) and $19 for the three months ended in 2024 and 2023, respectively	(36)	70
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 in 2024 and 2023, respectively	6	13
Net unrealized (loss) gain on available-for-sale investments	(30)	83
Other comprehensive income	74	217
Comprehensive income attributable to common stockholders	$ 100,972	$ 38,570