Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 59,684	$ 74,636	$ 91,761	$ 91,112	$ 97,417	$ 91,760
Unrecognized pension and post-retirement benefit costs:
Net (loss) gain arising during period	0	0	0	0	(20,053)	5,429
Prior service benefit	0	0	0	0	32,697	824
Reclassification adjustments included in net income for amortization of:
Prior service benefit	(2,414)	(1,663)	(7,243)	(4,993)	(9,657)	(6,636)
Net loss	1,695	1,087	5,083	3,532	6,516	5,688
Net unrealized gains on marketable securities:
Net holding gains arising during period	5,945	4,313	18,124	9,293	17,275	15,028
Reclassification adjustments for net gains included in net income	(1,765)	(2,072)	(9,122)	(5,570)	(11,192)	(8,797)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	134	126	396	371	523	490
Total other comprehensive income before income taxes	3,595	1,791	7,238	2,633	16,109	12,026
Income tax benefit (expense) related to items of other comprehensive (loss) income:
Unrecognized pension and post-retirement benefit costs	262	(228)	716	(6)	(3,539)	(2,536)
Net unrealized gains on marketable securities	(825)	(435)	(1,814)	(757)	(1,163)	(1,269)
Losses on cash flow hedges	(49)	(165)	(174)	(293)	(220)	(335)
Total income tax expense	(612)	(828)	(1,272)	(1,056)	(4,922)	(4,140)
Other comprehensive income, net of tax	2,983	963	5,966	1,577	11,187	7,886
Comprehensive income	$ 62,667	$ 75,599	$ 97,727	$ 92,689	$ 108,604	$ 99,646