Assets Held For Sale and Sales of Real Estate Narrative (Details) $ in Thousands			3 Months Ended		12 Months Ended
	Jan. 31, 2017 USD ($)	Jan. 26, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Long Lived Assets Held-for-sale [Line Items]
Assets Held for Sale, Number of Properties Sold				2
Proceeds from sale of real estate				$ 44,000
(Gain) loss on sales of real estate, net				$ (317)
Proceeds from FCC spectrum auction					$ 172,000
FCC Licenses
Long Lived Assets Held-for-sale [Line Items]
(Gain) Loss on Disposition of Intangible Assets			$ (133,000)
Denver, CO Property
Long Lived Assets Held-for-sale [Line Items]
Proceeds from sale of real estate		$ 23,000
Chicago, IL Property
Long Lived Assets Held-for-sale [Line Items]
Proceeds from sale of real estate	$ 22,000
Sale Leaseback Transaction, Lease Terms	P10Y
(Gain) loss on sales of real estate, deferred	$ (13,000)