STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 79,029	$ 81,977	$ 152,595	$ 141,303
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	2,676	37,451	(6,076)	5,215
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	3,463	2,660	6,926	5,320
Total other comprehensive income, net of tax	6,139	40,111	850	10,535
Comprehensive income	$ 85,168	$ 122,088	$ 153,445	$ 151,838