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, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (17,620)
Net current-period other comprehensive income (loss)	(6,406)	$ 7,144
Accumulated other comprehensive income (loss), ending balance	(24,026)		$ (17,620)	$ (17,620)
Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(14,656)	(16,454)		(16,454)
Other comprehensive income (loss) before reclassification				953
Amounts reclassified from accumulated other comprehensive income (loss)	182			845
Net current-period other comprehensive income (loss)	182			1,798
Accumulated other comprehensive income (loss), ending balance	(14,474)		(14,656)	(14,656)
Proportionate Share of Telesat Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(2,964)	(20,824)		(20,824)
Other comprehensive income (loss) before reclassification	(6,588)			22,033
Amounts reclassified from accumulated other comprehensive income (loss)
Net current-period other comprehensive income (loss)	(6,588)			22,033
Tax Cuts and Jobs Act, reclassification of tax effect from accumulated other comprehensive loss to accumulated deficit				(4,173)
Accumulated other comprehensive income (loss), ending balance	(9,552)		(2,964)	(2,964)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(17,620)	(37,278)		(37,278)
Other comprehensive income (loss) before reclassification	(6,588)			22,986
Amounts reclassified from accumulated other comprehensive income (loss)	182			845
Net current-period other comprehensive income (loss)	(6,406)	$ 7,144	16,687	23,831
Tax Cuts and Jobs Act, reclassification of tax effect from accumulated other comprehensive loss to accumulated deficit				(4,173)
Accumulated other comprehensive income (loss), ending balance	$ (24,026)		$ (17,620)	$ (17,620)