Description of business, basis of presentation and summary of significant accounting policies - Early Adoption of New Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Increase (decrease) in operating expenses	$ 1,357,124	$ 1,295,936	$ 1,134,528
Increase (decrease) in non-operating expenses	235,186	258,861	284,628
Revenues	1,903,026	2,004,088	1,764,822
Future minimum payments due under non-cancelable operating leases	109,353
Assets	4,962,115	8,542,725
Liabilities	3,967,074	5,943,455
New Accounting Pronouncement, Early Adoption, Effect \| Accounting Standards Update 2017-06
New Accounting Pronouncement, Early Adoption [Line Items]
Increase (decrease) in operating expenses	$ (6,700)	$ (7,600)	$ 700
Advertising Revenue \| Product Concentration Risk \| Sales Revenue
New Accounting Pronouncement, Early Adoption [Line Items]
Concentration risk (as a percent)	52.00%
Barter Arrangement, Syndicated Programming \| Product Concentration Risk \| Sales Revenue
New Accounting Pronouncement, Early Adoption [Line Items]
Revenues	$ 2,000
Retransmission Revenue \| Product Concentration Risk \| Sales Revenue
New Accounting Pronouncement, Early Adoption [Line Items]
Concentration risk (as a percent)	38.00%
Online Advertising Revenue \| Product Concentration Risk \| Sales Revenue
New Accounting Pronouncement, Early Adoption [Line Items]
Concentration risk (as a percent)	10.00%

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Total costs of sales and operating expenses for the period.

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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The aggregate amount of income or expense from ancillary business-related activities (that is to say, excluding major activities considered part of the normal operations of the business).

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Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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