STATEMENTS OF COMPREHENSIVE OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income	$ 98,261	$ 96,768	$ 81,918
Unrecognized pension and post-retirement benefit costs:
Net gain (loss) arising during period	12,634	(20,053)	5,429
Prior service benefit	0	32,697	824
Reclassification adjustments included in net income for amortization of:
Prior service benefit	(9,657)	(7,407)	(6,574)
Net loss	6,776	4,965	8,622
Net unrealized gains/losses on marketable securities:
Net holding gains (losses) arising during period	25,275	8,444	(2,906)
Reclassification adjustments for net gains included in net income	(10,626)	(7,640)	(11,114)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	532	498	467
Total other comprehensive income (loss) before income taxes	24,934	11,504	(5,252)
Income tax benefit (expense) related to items of other comprehensive income (loss):
Unrecognized pension and post-retirement benefit costs	(3,615)	(4,261)	(3,286)
Net unrealized (gains) losses on marketable securities	(2,922)	(106)	2,828
Losses on cash flow hedges	(223)	(339)	(203)
Total income tax expense	(6,760)	(4,706)	(661)
Other comprehensive income (loss), net of tax	18,174	6,798	(5,913)
Comprehensive income	$ 116,435	$ 103,566	$ 76,005