Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Net income	$ 70,667	$ 100,190	$ 102,430	$ 152,321
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, tax	36	36	72	73
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	111	112	223	223
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, tax	148	160	312	311
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax	461	457	906	923
Reclassification of postretirement liability adjustment from regulatory asset, tax	(1,086)	0	(1,086)	0
Reclassification of postretirement liability adjustment from regulatory asset, net of tax	(3,265)	0	(3,265)	0
Postretirement liability adjustment	(2,804)	457	(2,359)	923
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, tax	(34)	(13)	(119)	(25)
Net unrealized loss on available-for-sale investments arising during the period, net of tax	(128)	(52)	(448)	(96)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	7	6	15	15
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	25	25	57	60
Net unrealized loss on available-for-sale investments	(103)	(27)	(391)	(36)
Other comprehensive income (loss)	(2,796)	542	(2,527)	1,110
Comprehensive income attributable to common stockholders	$ 67,871	$ 100,732	$ 99,903	$ 153,431