NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Variable Interest Entities, Additional Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) license	Dec. 31, 2015 USD ($) license	Dec. 31, 2014 USD ($)
Variable Interest Entities
Revenue	$ 797,691	$ 693,835	$ 666,534	$ 578,889	$ 611,790	$ 548,404	$ 554,167	$ 504,775	$ 2,736,949	$ 2,219,136	$ 1,976,558
Income from equity and cost method investments									1,735	964	2,313
Capital lease additions										2,800	0
Eliminations
Variable Interest Entities
Revenue									(108,190)	(79,348)	(86,466)
Consolidated VIEs | Eliminations
Variable Interest Entities
Liabilities associated with the certain outsourcing agreements and purchase options	74,500				72,500				74,500	72,500
Consolidated VIEs | Cunningham | Eliminations
Variable Interest Entities
Total payments made under the LMA excluded from liabilities	40,800				37,600				40,800	37,600
Total capital leased assets excluded from VIE consolidation	4,500				4,500				$ 4,500	$ 4,500
Consolidated VIEs, aggregated
Variable Interest Entities
VIE licenses | license									37	37
Revenue									$ 310,400	$ 284,400	286,300
Consolidated VIEs, aggregated | Minimum
Variable Interest Entities
Outsourcing agreement initial term									5 years
VIEs which are not primary beneficiary
Variable Interest Entities
Carrying amount	$ 117,000				$ 18,100				$ 117,000	18,100
Income from equity and cost method investments									$ 2,500	$ 7,700	$ 2,200