Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 73,271	$ 59,684	$ 99,600	$ 91,761	$ 106,100	$ 97,417
Unrecognized pension and post-retirement benefit costs:
Net gain (loss) arising during period	0	0	0	0	12,634	(20,053)
Prior service benefit	0	0	0	0	0	32,697
Reclassification adjustments included in net income for amortization of:
Prior service benefit	(2,413)	(2,414)	(7,243)	(7,243)	(9,657)	(9,657)
Net loss	1,640	1,695	4,790	5,083	6,483	6,516
Net unrealized gains/loss on marketable securities:
Net holding gains (losses) arising during period	(948)	5,945	(4,909)	18,124	2,242	17,275
Reclassification adjustments for net (gains) losses included in net income	443	(1,765)	1,108	(9,122)	(396)	(11,192)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	144	134	423	396	559	523
Total other comprehensive income (loss) before income taxes	(1,134)	3,595	(5,831)	7,238	11,865	16,109
Income tax benefit (expense) related to items of other comprehensive income (loss):
Unrecognized pension and post-retirement benefit costs	174	262	520	716	(3,811)	(3,539)
Net unrealized (gains) losses on marketable securities	105	(825)	761	(1,814)	(347)	(1,163)
Losses on cash flow hedges	(32)	(49)	(113)	(174)	(162)	(220)
Total income tax benefit (expense)	247	(612)	1,168	(1,272)	(4,320)	(4,922)
Other comprehensive income (loss), net of tax	(887)	2,983	(4,663)	5,966	7,545	11,187
Comprehensive income	$ 72,384	$ 62,667	$ 94,937	$ 97,727	$ 113,645	$ 108,604