STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 73,566	$ 59,326
Other comprehensive loss, net of tax
Change in unrealized holding losses on available-for-sale securities	(8,752)	(32,236)
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	3,463	2,660
Total other comprehensive loss, net of tax	(5,289)	(29,576)
Comprehensive income	$ 68,277	$ 29,750