Acquisition and Disposition (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
	Mar. 16, 2015	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Recognized amounts of tangible assets and liabilities:
Goodwill (including deferred tax adjustment of $2,298)		$ 428,909	$ 428,909		$ 428,909	$ 398,164
Sale of business
Loss on sale			9,501		9,501
3Q Digital Inc
Acquisition and Disposition
Estimate of potential undiscounted future contingent consideration payments, low end of range	$ 0
Estimate of potential undiscounted future contingent consideration payments, high end of range	35,000
Total consideration
Cash consideration per purchase agreement	30,245
Estimated fair value of contingent consideration	17,940			$ 17,940
Fair value of total consideration transferred	48,185
Recognized amounts of tangible assets and liabilities:
Current assets	4,135
Property and equipment	164
Other assets	389
Current liabilities	(822)
Total tangible assets and liabilities:	3,866
Identifiable intangible assets	4,773
Goodwill (including deferred tax adjustment of $2,298)	41,844
Total	50,483
Deferred tax adjustment	2,298
Reconciliation of accrued earnout consideration
Accrued contingent consideration at beginning of period				17,940
Accrued contingent consideration at end of period	17,940
3Q Digital Inc | Customer relationships
Recognized amounts of tangible assets and liabilities:
Identifiable intangible assets	$ 4,300
Amortization period	7 years
3Q Digital Inc | Tradenames and trademarks
Recognized amounts of tangible assets and liabilities:
Identifiable intangible assets	$ 300
Amortization period	2 years
3Q Digital Inc | Non-compete agreements
Recognized amounts of tangible assets and liabilities:
Identifiable intangible assets	$ 200
Amortization period	3 years
3Q Digital Inc | Level 3 | Contingent Consideration
Total consideration
Estimated fair value of contingent consideration	$ 17,940	18,801	18,801	17,940	18,801
Reconciliation of accrued earnout consideration
Accrued contingent consideration at beginning of period				17,940
Accretion of interest				861
Accrued contingent consideration at end of period	$ 17,940	18,801	$ 18,801	$ 18,801	$ 18,801
B2B research business
Sale of business
Loss on sale		$ 9,500
B2B research business | Maximum
Sale of business
Percent of total Company revenue		5.00%