STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 89,216	$ 94,169	$ 230,519	$ 257,234
Other comprehensive income, net of tax
Change in unrealized holding gains on available-for-sale securities	6,143	2,330	11,358	10,387
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	2,659	1,231	7,979	3,694
Total other comprehensive income, net of tax	8,802	3,561	19,337	14,081
Comprehensive income	$ 98,018	$ 97,730	$ 249,856	$ 271,315