STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 94,169	$ 80,446	$ 257,234	$ 225,910
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	2,330	(492)	10,387	(6,470)
Amortization of prior service costs and net actuarial loss on pension and other postretirement plans	1,231	0	3,694	0
Total other comprehensive income (loss), net of tax	3,561	(492)	14,081	(6,470)
Comprehensive income	$ 97,730	$ 79,954	$ 271,315	$ 219,440