FAIR VALUE MEASUREMENTS - Fair Value, Assets and Liabilities, Reconciliation of Level 3 Inputs (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		$ 74,654			$ 74,654			$ 74,898	$ 82,966	$ 77,484	$ 81,439	$ 82,584
Gain (loss) from changes in fair value of investments		(244)	$ (3,955)		(8,312)	$ (5,100)
Unrealized Gains (Losses) Still Held - Assets	[1]	(244)	(3,955)		(8,312)	(5,100)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		1,543			1,543			1,542	0	0	0	14,588
Contingent consideration liability incurred in connection with acquisition of business		0	0		1,589	0
Business Combination, Contingent Consideration, Liability		0	0		0	(7,858)
Reclassification of contingent liability to non-fair value		0	0		0	(6,778)
Business Combination, Contingent Consideration Arrangements, Change in Range of Outcomes, Contingent Consideration, Liability, Value, Low		(20)		$ 0	(81)		$ 0
Purchases of Convertible Debt			0			1,612
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	[2]	21	0		35	48
Fair Value, Measurement with Unobservable Inputs, Unrealized Gain Loss	[1]	21	0		35	0
Redeemable Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		10,207			10,207			9,961	15,431	15,552	15,923	17,444
Unrealized Gains (Losses) Still Held - Assets	[1]	246	(371)		(5,224)	(1,892)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		246	(371)		246	(1,892)
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities		0	0		5,000	0
Convertible Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		0			0			$ 10,236	11,354	$ 11,232	$ 10,868	$ 10,038
Unrealized Gains (Losses) Still Held - Assets	[1]	0	364		0	1,180
Proceeds from Convertible Debt		(8,594)	0		(8,594)	(1,843)
available-for-sale investment transfer to other equity investment		(1,500)	0		(4,008)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		(106)	146		(1,148)	(387)
Unrealized Gain (Loss) on Securities		(36)	$ 218		2,396	$ 1,812
Convertible Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payments to Acquire Investments		0			0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		74,654			74,654				82,966
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		0			0				0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		0			0				0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value option investments		$ 74,654			$ 74,654				$ 82,966

[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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