Accounting Policies - Narrative (Details)	3 Months Ended			9 Months Ended
	Sep. 30, 2018 USD ($) market station	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment market station	Sep. 30, 2017 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Concentration Risk [Line Items]
Operating segments | segment				1
Reportable segments | segment				1
Number of television stations | station	47			47
Number of markets In which entity operates | market	39			39
Cumulative effect of new accounting principle in period of adoption, decrease in retained earnings	$ 3,724,000			$ 3,724,000
Revenues	538,976,000		$ 464,264,000	1,565,146,000	$ 1,412,703,000
Net cash provided by (used in) operating activities				332,660,000	351,739,000
Net cash provided by (used in) investing activities				(352,407,000)	153,366,000
Tax cuts and jobs act, reclassification from aoci to retained earnings, tax effect		$ 24,800,000
Restricted cash	0		$ 29,600,000	$ 0	29,600,000		$ 29,200,000	$ 28,200,000
Accounting Standards Update 2014-09 | Retained Earnings
Concentration Risk [Line Items]
Cumulative effect of new accounting principle in period of adoption, decrease in retained earnings						$ 3,700,000
Accounting Standards Update 2016-15
Concentration Risk [Line Items]
Net cash provided by (used in) operating activities					(900,000)
Net cash provided by (used in) investing activities					$ 900,000
Accounting Standards Update 2016-02
Concentration Risk [Line Items]
Estimated effect due to new accounting standard adoption, percentage				5.00%
Barter Arrangement, Syndicated Programming | Accounting Standards Update 2014-09
Concentration Risk [Line Items]
Revenues	$ 500,000			$ 1,500,000
Minimum
Concentration Risk [Line Items]
Revenue, performance obligation, description of timing				P60D
Maximum
Concentration Risk [Line Items]
Revenue, performance obligation, description of timing				P90D