Accounting Policies - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) market station	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2016 USD ($)
Concentration Risk [Line Items]
Number of television stations | station	47
Number of markets In which entity operates | market	39
Cumulative effect of new accounting principle in period of adoption			$ 3,724,000
Revenues	$ 502,090,000	$ 459,070,000
Net cash provided by (used in) operating activities	(51,186,000)	(139,917,000)
Tax cuts and jobs act, reclassification from aoci to retained earnings, tax effect	24,800,000
Restricted cash	0	$ 28,100,000	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				$ 3,700,000
Accounting Standards Update 2016-15
Concentration Risk [Line Items]
Net cash provided by (used in) operating 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			$ 2,000,000
Minimum
Concentration Risk [Line Items]
Revenue, performance obligation, description of timing	P60D
Maximum
Concentration Risk [Line Items]
Revenue, performance obligation, description of timing	P90D