STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 87,754	$ 79,706	$ 163,065	$ 145,464
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	2,579	(551)	8,057	(5,978)
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	1,231	0	2,463	0
Total other comprehensive income (loss), net of tax	3,810	(551)	10,520	(5,978)
Comprehensive income	$ 91,564	$ 79,155	$ 173,585	$ 139,486