Accumulated Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (71,972)
Other comprehensive income (loss), Net-of-tax Amount	(1,816)	$ 31,290
Accumulated other comprehensive income (loss), ending balance	(73,788)		$ (71,972)	$ (71,972)
Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(18,993)	(16,167)		(16,167)
Other comprehensive income (loss) before reclassification	0			(3,852)
Amounts reclassified from accumulated other comprehensive income (loss)	315			1,026
Other comprehensive income (loss), Net-of-tax Amount	315			(2,826)
Accumulated other comprehensive income (loss), ending balance	(18,678)		(18,993)	(18,993)
Equity in Telesat-related Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(52,979)	(38,747)		(38,747)
Other comprehensive income (loss) before reclassification	(2,131)			(14,232)
Amounts reclassified from accumulated other comprehensive income (loss)	0			0
Other comprehensive income (loss), Net-of-tax Amount	(2,131)			(14,232)
Accumulated other comprehensive income (loss), ending balance	(55,110)		(52,979)	(52,979)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(71,972)	(54,914)		(54,914)
Other comprehensive income (loss) before reclassification	(2,131)			(18,084)
Amounts reclassified from accumulated other comprehensive income (loss)	315			1,026
Other comprehensive income (loss), Net-of-tax Amount	(1,816)	$ 31,290	(48,348)	(17,058)
Accumulated other comprehensive income (loss), ending balance	$ (73,788)		$ (71,972)	$ (71,972)