STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 174,678	$ 167,505	$ 162,611
Other comprehensive (loss) income, net of tax
Change in unrealized holding (losses) gains on available-for-sale securities	(4,280)	823	(7,341)
Pension and other postretirement plans	25,117	(59,425)	81,433
Total other comprehensive income (loss), net of tax	20,837	(58,602)	74,092
Comprehensive income	$ 195,515	$ 108,903	$ 236,703