Investments in Affiliates (Narrative) (Details) $ / shares in Units, $ in Thousands, CAD in Millions		3 Months Ended						9 Months Ended			12 Months Ended
	Feb. 01, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Mar. 31, 2016 CAD shares	Mar. 31, 2016 USD ($) shares	Mar. 31, 2009 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) $ / shares		Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2015	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Investments in and Advances to Affiliates [Line Items]
Distribution received from affiliate								$ 242,735
Equity in net income (loss) of affiliates		$ 43,372		$ 6,948				183,086	$ 96,799
Net income (loss)		24,537		(3,892)				93,719	59,346
Proportionate share of Telesat other comprehensive income (loss), Before-Tax Amount		(19,255)		2,580				(24,184)	17,522	[1]
Proportionate share of Telesat other comprehensive income (loss)		(12,523)		1,572				(15,715)	10,863
Investments in affiliates		24,117						24,117				$ 107,950
Commitments to provide financial support
Equity method OCI previously suspended recovered									20,800
Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Economic interest in affiliates		62.70%						62.70%				62.70%		62.80%
Voting interest in affiliate		32.70%						32.70%				32.70%
Share appreciation rights exercised by Telesat employees | shares					178,642	178,642
Non-voting participating preferred stock issued to Telesat employees | shares					129,400	129,400
Equity method investee, stock options buy back, shares | shares					1,253,477	1,253,477
Equity method investee, stock options buy back, amount | CAD					CAD 24.7
Distribution received from affiliate			$ 242,700
Distributions declared to shareholders, option holders and certain employees			$ 400,000
Proportionate share in net income of Telesat		$ 81,600
Equity in net income (loss) of affiliates		43,372		$ 6,948				$ 183,086	96,799
Excess distribution over carrying value of investment in Telesat													$ 39,000
Investments in affiliates		24,117						24,117				$ 107,950
Equity method investment, excess equity income previously recognized recovered		39,000
Profits recognized on affiliate sales eliminated in earlier periods		800
Equity method losses previously suspended recovered						$ 57,900
Equity method OCI previously suspended recovered						$ 20,800
Telesat Holdings Inc [Member] | Equity Method Investment Term Loan [Member]
Investments in and Advances to Affiliates [Line Items]
Term loan borrowings		$ 2,430,000						$ 2,430,000
Debt Instrument, Offering Date								Nov. 17, 2016
Indenture maturity date								Nov. 17, 2023
Telesat Holdings Inc [Member] | Senior Notes 8.875% [Member]
Investments in and Advances to Affiliates [Line Items]
Interest on notes		8.875%						8.875%
Indenture maturity date								Nov. 17, 2024
Senior notes issued		$ 500,000						$ 500,000
Telesat Holdings Inc [Member] | Equity Method Investment Revolving Credit [Member]
Investments in and Advances to Affiliates [Line Items]
Revolving credit borrowings		$ 200,000						$ 200,000
Indenture maturity date								Nov. 17, 2021
Telesat Holdings Inc [Member] | 6% Senior Notes Due May 2017 [Member]
Investments in and Advances to Affiliates [Line Items]
Interest on notes		6.00%						6.00%
Debt instrument, repurchase date								Nov. 17, 2016
Senior notes issued		$ 0						$ 0
Telesat Holdings Inc [Member] | Equity Method Investment Senior Secured Credit Facility [Member]
Investments in and Advances to Affiliates [Line Items]
Minimum total leverage ratio		4.50						4.50
Actual Total Leverage Ratio to incur debt and make payments		4.64						4.64
Consulting fees payable in cash								$ 5,000
Telesat Holdings Inc [Member] | Equity Method Investment Senior Secured Facilities Repriced [Member]
Investments in and Advances to Affiliates [Line Items]
Term loan borrowings	$ 2,424,000
Debt instrument amendment date	Feb. 01, 2017
Telesat Holdings Inc [Member] | Management [Member]
Investments in and Advances to Affiliates [Line Items]
Equity method investee, stock options buy back, amount | CAD					CAD 18.7
Telesat Holdings Inc [Member] | Restatement Adjustment [Member] | Overstatement [Member]
Investments in and Advances to Affiliates [Line Items]
Equity in net income (loss) of affiliates									3,000
Net income (loss)									$ 1,800
Earnings per share, basic and diluted | $ / shares									$ 0.06
XTAR, LLC [Member]
Investments in and Advances to Affiliates [Line Items]
Investments in affiliates		$ 0						$ 0				0
Percentage of ownership interest		56.00%						56.00%
Amended lease obligation, annual												18,200
Lease agreement, past due																$ 32,300
Lease agreement, deferred amount															$ 6,700
Repayment term past due and deferred lease obligation, years							12 years
Deferred lease obligation, annual payment							$ 5,000
Discount rate for prepayment of restructured past due and deferred lease obligation							9.00%
Cumulative payments of restructured past due and deferred lease obligation								$ 29,200
Equity method investment, investee past due liability		$ 31,700						$ 31,700				28,800
XTAR, LLC [Member] | Scenario, Forecast [Member]
Investments in and Advances to Affiliates [Line Items]
Maximum lease obligation, annual											$ 28,000
XTAR, LLC [Member] | Scenario, Plan [Member]
Investments in and Advances to Affiliates [Line Items]
Lease obligation, Annual											26,000	$ 26,000
Amended lease obligation, annual											$ 9,500

[1] Includes $20.8 million ($13.5 million, net of tax) share in the equity of Telesat's other comprehensive income that we could not record in 2015 (see Note 5)