SEGMENT INFORMATION - Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment information
Operating income	$ 1,198,787	$ 1,085,338	$ 893,653
Stock-based compensation expense	52,300	45,800	32,700
Corporate and unallocated
Segment information
Operating income	(245,833)	(228,505)	(197,474)
Payroll costs	156,300	128,000	109,800
Stock-based compensation expense	52,300	45,800	32,700
Professional service expenses	51,800	66,300	60,800
Insurance costs	6,000	6,000	7,000
Other operating expenses	$ 31,700	$ 28,200	$ 19,900