STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 81,977	$ 87,754	$ 141,303	$ 163,065
Other comprehensive income, net of tax
Change in unrealized holding gains on available-for-sale securities	37,451	2,579	5,215	8,057
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	2,660	1,231	5,320	2,463
Total other comprehensive income, net of tax	40,111	3,810	10,535	10,520
Comprehensive income	$ 122,088	$ 91,564	$ 151,838	$ 173,585