Acquisition of Art Agency, Partners (Details)		3 Months Ended				12 Months Ended
	Jan. 11, 2016 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) annual_incremental_payment
Business Acquisition [Line Items]
Acquisition earn-out consideration (see Note 3)						$ 17,500,000	$ 26,250,000
Allocation of purchase price:
Goodwill					$ 13,621,000	50,547,000	50,029,000
Period over which goodwill is tax deductible		15 years
Agency
Allocation of purchase price:
Goodwill					13,621,000	44,396,000	43,878,000
All Other
Allocation of purchase price:
Goodwill					0	6,151,000	6,151,000
Art Agency, Partners
Purchase price:
Initial cash consideration	$ 50,000,000
Working capital adjustment	1,189,000
Total purchase price	51,189,000
Allocation of purchase price:
Net working capital acquired	1,572,000
Fixed assets and other long-term assets	173,000
Goodwill	34,490,000
Deferred tax assets	1,094,000
Total purchase price	51,189,000
Goodwill acquired						0	34,490,000
Transaction costs	800,000
Art Agency, Partners | General and Administrative Expenses
Allocation of purchase price:
Transaction expenses				$ (200,000)	$ (600,000)
Art Agency, Partners | Agency
Allocation of purchase price:
Goodwill acquired		$ 28,300,000				0	28,339,000
Art Agency, Partners | All Other
Allocation of purchase price:
Goodwill acquired		$ 6,200,000				0	6,151,000
Art Agency, Partners | Customer relationships
Allocation of purchase price:
Intangible assets	10,800,000
Art Agency, Partners | Employment contracts
Allocation of purchase price:
Intangible assets	$ 3,060,000
Art Agency, Partners | Former Owner
Business Acquisition [Line Items]
Employment agreement term	5 years
Non-compete and non-solicit covenants period after employment agreement	12 months
Art Agency, Partners | Earn Out
Business Acquisition [Line Items]
Compensation expense related to earn-out arrangement							$ 35,000,000
Settlement period for earn-out payments	4 years
Number of annual incremental payments | annual_incremental_payment							4
Maximum annual payments under contingent earn-out agreement			$ 8,750,000
Contingent consideration, liability, current						8,750,000
Acquisition earn-out consideration (see Note 3)						$ 17,500,000
Art Agency, Partners | Earn Out | Agency
Business Acquisition [Line Items]
Compensation expense related to earn-out arrangement							$ 23,900,000
Art Agency, Partners | Earn Out | All Other
Business Acquisition [Line Items]
Compensation expense related to earn-out arrangement							$ 11,100,000