Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net income	$ 378,131	$ 390,205	$ 335,453
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, tax	145	145	(140)
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	446	446	731
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, tax	1,626	(2,606)	(2,012)
Postretirment liability gains (losses) arising during the period, net of tax	4,876	(8,395)	(6,151)
Amortization of postretirement liability losses in net periodic benefit cost, tax	615	630	476
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	1,870	1,922	1,486
Postretirement liability adjustment	6,746	(6,473)	(4,665)
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(67)	0	35
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(252)	(1)	134
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	36	14	10
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	134	52	40
Net unrealized gain (loss) on available-for-sale investments	(118)	51	174
Other comprehensive income (loss)	7,074	(5,976)	(3,760)
Comprehensive income attributable to common stockholders	$ 385,205	$ 384,229	$ 331,693