Goodwill and other intangible assets - Narrative (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges		$ 15,218	$ 8,000	$ 30,300
Digital
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges	$ 15,200	15,218	8,000
Operating Segments | Digital
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges	$ 15,200	15,218	$ 8,000	$ 30,271
Aurico Inc.
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		$ 14,100
Weighted average useful life		8 years
DealerRater
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life		10 years
DealerRater | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		$ 24,700
DealerRater | Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		$ 14,100
Workterra LLC
Finite-Lived Intangible Assets [Line Items]
Weighted average useful life		8 years
Workterra LLC | Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired		$ 13,700