Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Net income	$ 31,763	$ 52,131
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	36	37
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	112	111
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, tax	164	151
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax	445	466
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, tax	(85)	(12)
Net unrealized loss on available-for-sale investments arising during the period, net of tax	(320)	(44)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	8	9
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	32	35
Net unrealized loss on available-for-sale investments	(288)	(9)
Other comprehensive income	269	568
Comprehensive income attributable to common stockholders	$ 32,032	$ 52,699