Goodwill and intangible assets - Other Amortizable Intangible (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Changes in other amortizable intangible assets
Balance at the beginning of the period			$ 34,975
Additions			65
Amortization	$ (3,300)	$ (3,300)	(9,936)	$ (10,000)
Net balance at the end of the period	25,104		25,104
Future estimated amortization expense
Remainder of 2018					$ 3,311
2019					8,508
2020					6,080
2021					5,767
2022 and thereafter					1,438
Total	25,104		$ 34,975		25,104
Remaining amortization period			3 years
Affiliate relationships
Changes in other amortizable intangible assets
Balance at the beginning of the period			$ 32,343
Amortization			(9,053)
Net balance at the end of the period	23,290		23,290
Future estimated amortization expense
Total	23,290		32,343		23,290
Advertiser relationships
Changes in other amortizable intangible assets
Balance at the beginning of the period			1,240
Amortization			(412)
Net balance at the end of the period	828		828
Future estimated amortization expense
Total	828		1,240		828
Non-compete agreement
Changes in other amortizable intangible assets
Balance at the beginning of the period			1,235
Amortization			(412)
Net balance at the end of the period	823		823
Future estimated amortization expense
Total	823		1,235		823
Other intangibles
Changes in other amortizable intangible assets
Balance at the beginning of the period			157
Additions			65
Amortization			(59)
Net balance at the end of the period	163		163
Future estimated amortization expense
Total	$ 163		$ 157		$ 163