ACQUISITIONS - CastleLine (Details) - USD ($) shares in Thousands		12 Months Ended
	Jul. 17, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2016
Acquisitions
Business combination, consideration transferred		$ 0
Value of common stock paid at acquisition		0	$ 0	$ 21,733,000
Goodwill		86,283,000	$ 86,283,000	$ 82,801,000
CastleLine
Acquisitions
Cash paid at acquisition	$ 12,300,000
Contingent consideration, cash payable	$ 10,500,000
Contingent consideration arrangements, payment period	4 years
Number of shares exchanged for acquisition (in shares)	495
Value of common stock paid at acquisition	$ 14,400,000
Business combination, contingent consideration, liability, noncurrent, payments		8,900,000
Cash					$ 1,088,000
Accounts receivable, net					100,000
Prepaid expenses					20,000
Restricted cash					2,501,000
Non-compete agreements					1,130,000
Databases/other					1,800,000
Customer relationships					395,000
Trademarks and trade names					160,000
Deferred taxes					356,000
Goodwill					26,730,000
Assets acquired					34,280,000
Accounts payable and accrued expenses					(837,000)
Deferred revenue					0
Purchase price					33,443,000
CastleLine | Initial purchase price allocation
Acquisitions
Business combination, consideration transferred	33,400,000
Cash	1,088,000
Accounts receivable, net	510,000
Prepaid expenses	66,000
Restricted cash	2,501,000
Non-compete agreements	1,105,000
Databases/other	465,000
Customer relationships	395,000
Trademarks and trade names	150,000
Deferred taxes	0
Goodwill	28,125,000
Assets acquired	34,405,000
Accounts payable and accrued expenses	(875,000)
Deferred revenue	(87,000)
Purchase price	33,443,000
CastleLine | Adjustments
Acquisitions
Cash					0
Accounts receivable, net					(410,000)
Prepaid expenses					(46,000)
Restricted cash					0
Non-compete agreements					25,000
Databases/other					1,335,000
Customer relationships					0
Trademarks and trade names					10,000
Deferred taxes					356,000
Goodwill					(1,395,000)
Assets acquired					(125,000)
Accounts payable and accrued expenses					38,000
Deferred revenue					87,000
Purchase price					$ 0
Contingent on Future Employment | CastleLine
Acquisitions
Additional consideration	$ 3,800,000
Business combination, contingent consideration, liability, noncurrent, payments		$ 2,100,000