Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Net income	$ 38,353	$ 31,763
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	11	36
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	34	112
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, tax	34	164
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax	100	445
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	19	(85)
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	70	(320)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	3	8
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	13	32
Net unrealized gain (loss) on available-for-sale investments	83	(288)
Other comprehensive income	217	269
Comprehensive income attributable to common stockholders	$ 38,570	$ 32,032