Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net income	$ 147,971	$ 139,276	$ 250,401	$ 291,597
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, tax	37	36	109	109
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	111	111	334	334
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit credit, tax	148	151	460	462
Amortization of postretirement liability losses included in net periodic benefit credit, net of tax	461	466	1,367	1,389
Reclassification of postretirement liability adjustment from regulatory asset, tax	0	0	(1,086)	0
Reclassification of postretirement liability adjustment from regulatory asset, net of tax	0	0	(3,265)	0
Postretirement liability adjustment	461	466	(1,898)	1,389
Net unrealized loss on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, tax	(88)	(15)	(207)	(40)
Net unrealized loss on available-for-sale investments arising during the period, net of tax	(329)	(54)	(777)	(150)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	12	6	27	21
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	42	20	99	80
Net unrealized loss on available-for-sale investments	(287)	(34)	(678)	(70)
Other comprehensive income (loss)	285	543	(2,242)	1,653
Comprehensive income attributable to common stockholders	$ 148,256	$ 139,819	$ 248,159	$ 293,250