Accumulated Other Comprehensive Loss (Components of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated other comprehensive income (loss), beginning balance			$ (13,836)
Net current-period other comprehensive income (loss)	$ (12,356)	$ 1,713	(15,215)	$ 11,289
Accumulated other comprehensive income (loss), ending balance	(29,051)		(29,051)		$ (13,836)
Postretirement Benefits [Member]
Accumulated other comprehensive income (loss), beginning balance			(14,074)	(13,459)	(13,459)
Other comprehensive income (loss) before reclassification					(1,209)
Amounts reclassified from accumulated other comprehensive income (loss)			500		594
Net current-period other comprehensive income (loss)			500		(615)
Accumulated other comprehensive income (loss), ending balance	(13,574)		(13,574)		(14,074)
Proportionate Share of Telesat Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income (loss), beginning balance			238	(15,239)	(15,239)
Other comprehensive income (loss) before reclassification			(15,715)		15,477
Amounts reclassified from accumulated other comprehensive income (loss)
Net current-period other comprehensive income (loss)			(15,715)		15,477
Accumulated other comprehensive income (loss), ending balance	(15,477)		(15,477)		238
Accumulated Other Comprehensive Loss [Member]
Accumulated other comprehensive income (loss), beginning balance			(13,836)	$ (28,698)	(28,698)
Other comprehensive income (loss) before reclassification			(15,715)		14,268
Amounts reclassified from accumulated other comprehensive income (loss)			500		594
Net current-period other comprehensive income (loss)			(15,215)		14,862
Accumulated other comprehensive income (loss), ending balance	$ (29,051)		$ (29,051)		$ (13,836)