FAIR VALUE MEASUREMENTS - Fair Value, Assets and Liabilities, Reconciliation of Level 3 Inputs (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		$ 74,898	$ 81,439		$ 74,898	$ 81,439		$ 77,933	$ 82,966	$ 82,887	$ 82,584
Gain (loss) from changes in fair value of investments		(3,035)	(1,448)		(8,068)	(1,145)
Unrealized Gains (Losses) Still Held - Assets	[1]	(3,035)	(1,448)		(8,068)	(1,145)
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		1,542	0		1,542	0		0	0	14,600	14,588
Contingent consideration liability incurred in connection with acquisition of business		1,589	0		1,589	0
Business Combination, Contingent Consideration, Liability		0	(7,858)		0	(7,858)
Reclassification of contingent liability to non-fair value		0	(6,778)		0	(6,778)
Business Combination, Contingent Consideration Arrangements, Change in Range of Outcomes, Contingent Consideration, Liability, Value, Low		(61)		$ 0	(61)		$ 0
Purchases of Convertible Debt			0			1,612
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities		4,600			5,000
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	[2]	14	36		14	48
Fair Value, Measurement with Unobservable Inputs, Unrealized Gain Loss	[1]	(47)	0		(47)	0
Redeemable Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		9,961	15,923		9,961	15,923		14,576	15,431	17,788	17,444
Unrealized Gains (Losses) Still Held - Assets	[1]	(4,615)	(1,865)		(5,470)	(1,521)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	(1,865)		0	(1,521)
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities		4,615	0		5,000	0
Convertible Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		10,236	10,868		10,236	10,868		$ 11,070	$ 11,354	$ 11,931	$ 10,038
Unrealized Gains (Losses) Still Held - Assets	[1]	2,599	512		2,405	816
Proceeds from Convertible Debt		0	(1,843)		0	(1,843)
available-for-sale investment transfer to other equity investment		(2,508)	0		(2,508)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(541)	(575)		(1,042)	(533)
Unrealized Gain (Loss) on Securities		2,215	$ 1,355		2,432	$ 1,594
Convertible Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payments to Acquire Investments		$ 0			$ 0

[1] (1)Represents the unrealized gains or losses recorded in earnings and/or other comprehensive income (loss) during the period for assets and liabilities classified as Level 3 that are still held (or outstanding) at the end of the period.

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