Asset impairment and other (gains) charges - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unusual or Infrequent Item, or Both [Line Items]
Gain on sale of real estate					$ 6,000
Asset impairment charges						$ 2,200	$ 4,700
Reimbursement from spectrum repacking					7,400	$ 0	0
Goodwill impairment charges			$ 332,900				15,200
Hurricane Harvey
Unusual or Infrequent Item, or Both [Line Items]
Net expense impact from hurricane		$ 900		$ 900	1,100
Gross hurricane related expenses		26,900		15,800
Insurance recovery	$ 21,000	$ 5,000		$ 26,000	$ 26,000
Cofactor Reporting Unit
Unusual or Infrequent Item, or Both [Line Items]
Goodwill impairment charges							15,200
Internally produced program
Unusual or Infrequent Item, or Both [Line Items]
Asset impairment charges							$ 6,300