STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 316,821	$ 288,224	$ 196,999
Other comprehensive income (loss), net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	11,718	(9,937)	(190)
Pension and other postretirement plans	1,698	35,712	(8,105)
Total other comprehensive income (loss), net of tax	13,416	25,775	(8,295)
Comprehensive income	$ 330,237	$ 313,999	$ 188,704