Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income	$ 33,295	$ 36,066	$ 26,329	$ 32,077	$ 92,513	$ 112,369
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	(2,414)	(2,413)	(4,830)	(4,829)	(9,658)	(8,906)
Net loss	1,575	1,694	3,150	3,388	6,538	5,908
Net unrealized gains/loss on marketable securities:
Net holding gains (losses) arising during period	(1,253)	4,458	(3,961)	12,179	9,135	15,643
Reclassification adjustments for net (gains) losses included in net income	147	(5,166)	665	(7,357)	(2,604)	(11,499)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	140	132	279	262	549	515
Total other comprehensive income (loss) before income taxes	(1,805)	(1,295)	(4,697)	3,643	16,594	14,305
Income tax benefit (expense) related to items of other comprehensive income (loss):
Unrecognized pension and post-retirement benefit costs	190	261	346	454	(3,723)	(4,029)
Net unrealized (gains) losses on marketable securities	221	132	656	(989)	(1,277)	(773)
Losses on cash flow hedges	(31)	(47)	(81)	(125)	(179)	(336)
Total income tax benefit (expense)	380	346	921	(660)	(5,179)	(5,138)
Other comprehensive income (loss), net of tax	(1,425)	(949)	(3,776)	2,983	11,415	9,167
Comprehensive income	$ 31,870	$ 35,117	$ 22,553	$ 35,060	$ 103,928	$ 121,536