BUSINESS ACQUISITIONS - WRB Communications, LLC (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	May 08, 2017	May 05, 2017	Dec. 31, 2017	Dec. 31, 2018	Jan. 31, 2018	Dec. 31, 2016	Dec. 31, 2014
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 7,730
Goodwill			$ 832,624	$ 609,592		$ 316,616	$ 256,318
WRB Communications, LLC
BUSINESS ACQUISITIONS
Cash	26,804
Restricted common shares	4,291
Contingent consideration at fair value	530
Total	$ 31,625
Restricted common shares (in shares)	299,325
Market price (in dollars per share)		$ 15.93
Market price multiplier to factor in restricted nature of the shares (as a percent)		90.00%
Maximum additional cash payouts per performance period	$ 500
Amount received by the formers owners					$ 1,000
Purchase consideration deposited into an escrow account	$ 1,950
Deposit term into an escrow account	18 months
Acquisition-related costs charged to Selling, general, and administrative expenses			$ 259
Summary of the preliminary fair value determination of the acquired assets and liabilities
Cash	$ 1,018
Accounts receivable	2,593
Prepaid expenses and other current assets	179
Property and equipment	498
Definite-lived intangible assets	7,730
Other noncurrent assets	24
Accounts payable	(100)
Accrued expenses - other	(498)
Total identifiable net assets	11,444
Goodwill	20,181
Total acquisition price	31,625
Customer relationships | WRB Communications, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 5,200
Definite-lived intangible assets
Useful Life	7 years
Non-compete employment agreements | WRB Communications, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 1,530
Definite-lived intangible assets
Useful Life	4 years
Trade names and trademarks | WRB Communications, LLC
Summary of the preliminary fair value determination of the acquired assets and liabilities
Definite-lived intangible assets	$ 1,000
Definite-lived intangible assets
Useful Life	2 years