Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Net income	$ 81,965	$ 100,898
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax of $18 and $34 for the three months ended in 2025 and 2024, respectively	118	104
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 $19 and $(10) for the three months ended in 2025 and 2024, respectively	71	(36)
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 in 2025 and 2024, respectively	3	6
Net unrealized gain (loss) on available-for-sale investments	74	(30)
Other comprehensive income	192	74
Comprehensive income attributable to common stockholders	$ 82,157	$ 100,972