STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 293,304	$ 316,821	$ 288,224
Other comprehensive income, net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	18,738	11,718	(9,937)
Pension and other postretirement plans	19,987	1,698	35,712
Total other comprehensive income, net of tax	38,725	13,416	25,775
Comprehensive income	$ 332,029	$ 330,237	$ 313,999