Summary of Significant Accounting Policies - Goodwill (Details)	12 Months Ended
Dec. 31, 2017 market unit
Goodwill [Line Items]
Number of distinct markets \| market	67
Local Marketing Solutions
Goodwill [Line Items]
Number of reporting units \| unit	1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_GoodwillLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration