Investments in Affiliates (Narrative) (Details) $ in Thousands, CAD in Millions		3 Months Ended					12 Months Ended
	Feb. 01, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 CAD shares	Mar. 31, 2016 USD ($) shares		Mar. 31, 2009 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Investments in and Advances to Affiliates [Line Items]
Equity in net income (loss) of affiliates		$ 139,714		$ 46,494
Net income (loss)		71,359		30,686
Other Comprehensive Income Proportionate Share Equity Method Investee Before Tax Portion Attributable To Parent		(4,929)		14,448	[1]
Other Comprehensive Income Proportionate Share Equity Method Investee Net Of Tax Portion Attributable To Parent		(3,192)		9,032
Distribution received from affiliate		$ 242,735
Investments in affiliates								$ 107,950
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%							62.80%
Voting interest in affiliate		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
Equity in net income (loss) of affiliates		$ 139,714		$ 46,494
Distribution received from affiliate		242,700
Distributions declared to shareholders, option holders and certain employees		400,000
Investments in affiliates		0						107,950
Equity method investment, excess cash distribution recognized as equity income		103,800
Equity method investment, excess cash distribution recoverable from future equity in investee earnings		103,800
Equity method losses previously suspended recovered				57,900
Equity method OCI previously suspended recovered				20,800
Consulting fees payable in cash		5,000
Capital expenditures		$ 34,300		$ 69,200
XTAR, LLC [Member]
Investments in and Advances to Affiliates [Line Items]
Percentage of ownership interest		56.00%
Maximum lease obligation, annual							$ 28,000	28,000
Lease obligation, Annual							26,000	26,000
Amended lease obligation, annual							$ 9,500	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		$ 29,800						$ 28,800
6% Senior Notes [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Debt Instrument, Repurchase Date		Nov. 17, 2016
Senior notes issued		$ 0
Senior Notes 8.875% [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Interest on notes		8.875%
Indenture maturity date		Nov. 17, 2024
Senior notes issued		$ 500,000
Equity Method Investment Term Loan [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Term loan borrowings		$ 2,430,000
Indenture maturity date		Nov. 17, 2023
Equity Method Investment Revolving Credit [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Revolving credit borrowings		$ 200,000
Indenture maturity date		Nov. 17, 2021
Equity Method Investment Senior Secured Credit Facility [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Minimum Total Leverage Ratio		4.50
Actual Total Leverage Ratio to incur debt and make payments		4.86
Equity Method Investment Senior Secured Facilities Repriced [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Term loan borrowings	$ 2,424,000
Debt instrument amendment date	Feb. 01, 2017
Management [Member] | Telesat Holdings Inc [Member]
Investments in and Advances to Affiliates [Line Items]
Equity method investee, stock options buy back, amount | CAD			CAD 18.7

[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)