Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total	$ 89,990	$ 87,952	$ 88,566	$ 89,528	$ 95,163	$ 94,300	$ 96,888	$ 102,294	$ 356,036	[1]	$ 388,645	[1]	$ 419,482	[1]
Products and services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total									77,398		93,036		113,583
Products and services transferred over time
Disaggregation of Revenue [Line Items]
Total									278,638		295,609		305,899
Ratings and Planning
Disaggregation of Revenue [Line Items]
Total									253,652		271,623		285,355
Analytics and Optimization
Disaggregation of Revenue [Line Items]
Total									69,080		74,725		92,380
Movies Reporting and Analytics
Disaggregation of Revenue [Line Items]
Total									33,304		42,297		41,747
Reportable Geographical Components | United States
Disaggregation of Revenue [Line Items]
Total									310,717		336,087		359,379
Reportable Geographical Components | Europe
Disaggregation of Revenue [Line Items]
Total									27,447		30,619		34,623
Reportable Geographical Components | Latin America
Disaggregation of Revenue [Line Items]
Total									6,275		10,326		13,179
Reportable Geographical Components | Canada
Disaggregation of Revenue [Line Items]
Total									7,046		7,046		7,882
Reportable Geographical Components | Other
Disaggregation of Revenue [Line Items]
Total									$ 4,551		$ 4,567		$ 4,419

[1] Transactions with related parties are included in the line items above as follows (refer to Footnote 13, Related Party Transactions, of the Notes to Consolidated Financial Statements for additional information): Years Ended December 31,202020192018Revenues$13,314 $17,464 $12,662 Cost of revenues10,094 10,490 11,239 General and administrative316 776 650 Interest expense, net(24,480)(23,494)(16,023)