Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income	$ 153,078	$ 137,637	$ 277,911	$ 240,388
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	37	36	109	(177)
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	111	112	334	620
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, tax	152	95	456	284
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	471	292	1,413	878
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(13)	3	19	35
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(50)	12	72	130
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, tax	7	(1)	9	10
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, net of tax	28	(4)	34	36
Net unrealized gain (loss) on available-for-sale investments	(22)	8	106	166
Other comprehensive income	560	412	1,853	1,664
Comprehensive income attributable to common stockholders	$ 153,638	$ 138,049	$ 279,764	$ 242,052