STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 196,999	$ 210,366	$ 174,678
Other comprehensive (loss) income, net of tax
Change in unrealized holding (losses) gains on available-for-sale securities	(190)	43	(4,280)
Pension and other postretirement plans	(8,105)	(24,560)	25,117
Total other comprehensive (loss) income, net of tax	(8,295)	(24,517)	20,837
Comprehensive income	$ 188,704	$ 185,849	$ 195,515