Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net income	$ 139,276	$ 153,078	$ 291,597	$ 277,911
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	36	37	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 cost, tax	151	152	462	456
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	466	471	1,389	1,413
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(15)	(13)	(40)	19
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(54)	(50)	(150)	72
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	6	7	21	9
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	20	28	80	34
Net unrealized gain (loss) on available-for-sale investments	(34)	(22)	(70)	106
Other comprehensive income	543	560	1,653	1,853
Comprehensive income attributable to common stockholders	$ 139,819	$ 153,638	$ 293,250	$ 279,764