Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 390,205	$ 335,453	$ 272,318
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, tax	145	(140)	429
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	446	731	162
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, tax	(2,606)	(2,012)	1,471
Postretirment liability gains (losses) arising during the period, net of tax	(8,395)	(6,151)	4,441
Amortization of postretirement liability losses in net periodic benefit cost, tax	630	476	721
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	1,922	1,486	2,173
Postretirement liability adjustment	(6,473)	(4,665)	6,614
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, tax	0	0	(14)
Foreign currency translation adjustment recognized during the period, net of tax	0	0	(61)
Reclassification adjustment for foreign currency translation adjustment included in net income, tax	0	0	75
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax	0	0	249
Foreign currency translation adjustment	0	0	188
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	0	35	(38)
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(1)	134	(144)
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	14	10	35
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	52	40	131
Net unrealized gain (loss) on available-for-sale investments	51	174	(13)
Other comprehensive income (loss)	(5,976)	(3,760)	6,951
Comprehensive income attributable to common stockholders	$ 384,229	$ 331,693	$ 279,269