Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net income	$ 52,131	$ 25,130
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	37	36
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	111	111
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, tax	151	149
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	466	462
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(12)	36
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(44)	135
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, tax	9	0
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, net of tax	35	(1)
Net unrealized gain (loss) on available-for-sale investments	(9)	134
Other comprehensive income	568	707
Comprehensive income attributable to common stockholders	$ 52,699	$ 25,837