Goodwill and intangible assets - Other Amortizable Intangible (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018
Changes in other amortizable intangible assets
Balance at the beginning of the period	$ 34,975
Additions	33
Amortization	(3,314)	$ (3,300)
Net balance at the end of the period	31,694
Future estimated amortization expense
Remainder of 2018			$ 9,933
2019			8,497
2020			6,069
2021			5,757
2022 and thereafter			1,438
Total	34,975		31,694
Affiliate relationships
Changes in other amortizable intangible assets
Balance at the beginning of the period	32,343
Amortization	(3,017)
Net balance at the end of the period	29,326
Future estimated amortization expense
Total	32,343		29,326
Advertiser relationships
Changes in other amortizable intangible assets
Balance at the beginning of the period	1,240
Amortization	(138)
Net balance at the end of the period	1,102
Future estimated amortization expense
Total	1,240		1,102
Non-compete agreement
Changes in other amortizable intangible assets
Balance at the beginning of the period	1,235
Amortization	(137)
Net balance at the end of the period	1,098
Future estimated amortization expense
Total	1,235		1,098
Other intangibles
Changes in other amortizable intangible assets
Balance at the beginning of the period	157
Additions	33
Amortization	(22)
Net balance at the end of the period	168
Future estimated amortization expense
Total	$ 157		$ 168