Business operations and segment information - Narrative (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) market station	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of television stations \| station	47
Number of markets In which entity operates \| market	39
Revenues	$ 1,903,026	$ 2,004,088	$ 1,764,822
Operating Segments \| Media
Segment Reporting Information [Line Items]
Revenues	$ 1,903,026	$ 1,994,120	$ 1,713,982

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).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration