STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 210,366	$ 174,678	$ 167,505
Other comprehensive (loss) income, net of tax
Change in unrealized holding gains (losses) on available-for-sale securities	43	(4,280)	823
Pension and other postretirement plans	(24,560)	25,117	(59,425)
Total other comprehensive (loss) income, net of tax	(24,517)	20,837	(58,602)
Comprehensive income	$ 185,849	$ 195,515	$ 108,903