Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net income	$ 36,066	$ 22,284	$ 32,077	$ 16,476	$ 112,369	$ 73,864
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	(2,413)	(1,664)	(4,829)	(3,330)	(8,906)	(6,579)
Net loss	1,694	1,222	3,388	2,445	5,908	6,567
Net unrealized gains on marketable securities:
Net holding gains arising during period	4,458	2,790	12,179	4,980	15,643	2,623
Reclassification adjustments for net gains included in net income	(5,166)	(2,110)	(7,357)	(3,498)	(11,499)	(11,049)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	132	123	262	245	515	482
Total other comprehensive (loss) income before income taxes	(1,295)	361	3,643	842	14,305	(1,703)
Income tax benefit (expense) related to items of other comprehensive (loss) income:
Unrecognized pension and post-retirement benefit costs	261	166	454	222	(4,029)	(2,442)
Net unrealized losses (gains) on marketable securities	132	(149)	(989)	(322)	(773)	1,625
Losses on cash flow hedges	(47)	(46)	(125)	(128)	(336)	(216)
Total income tax benefit (expense)	346	(29)	(660)	(228)	(5,138)	(1,033)
Other comprehensive (loss) income, net of tax	(949)	332	2,983	614	9,167	(2,736)
Comprehensive income	$ 35,117	$ 22,616	$ 35,060	$ 17,090	$ 121,536	$ 71,128