Statements Of Comprehensive Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Net income (loss)	$ (6,966)	$ (3,989)	$ 95,285	$ 98,587
Unrecognized pension and post-retirement benefit costs:
Net gain (loss) arising during period	0	0	12,634	(20,053)
Prior service benefit	0	0	0	32,697
Reclassification adjustments included in net income for amortization of:
Prior service benefit	(2,416)	(2,416)	(9,657)	(8,157)
Net loss	1,575	1,694	6,657	5,436
Net unrealized gains/loss on marketable securities:
Net holding gains (losses) arising during period	(2,708)	7,721	14,846	13,975
Reclassification adjustments for net (gains) losses included in net income	518	(2,191)	(7,917)	(8,443)
Net losses on cash flow hedges:
Reclassification adjustment for interest expense included in net income	139	130	541	506
Total other comprehensive income (loss) before income taxes	(2,892)	4,938	17,104	15,961
Income tax benefit (expense) related to items of other comprehensive income (loss):
Unrecognized pension and post-retirement benefit costs	156	193	(3,652)	(4,124)
Net unrealized (gains) losses on marketable securities	435	(1,121)	(1,366)	(1,054)
Losses on cash flow hedges	(50)	(78)	(195)	(335)
Total income tax benefit (expense)	541	(1,006)	(5,213)	(5,513)
Other comprehensive income (loss), net of tax	(2,351)	3,932	11,891	10,448
Comprehensive income (loss)	$ (9,317)	$ (57)	$ 107,176	$ 109,035