LONG-TERM DEBT - Credit Facility Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Abstract]
Credit facility outstanding	$ 255,000	$ 0
Available for borrowings	795,000	1,050,000
Total borrowing commitments	1,050,000	1,050,000
Maximum amount outstanding at any month-end	285,500	214,500	$ 685,000
Average daily amount outstanding	$ 131,822	$ 33,642	$ 358,929
Average daily amount outstanding (in percentage)	2.54%	1.72%	1.60%
Amount outstanding at year-end (in percentage)	2.77%	0.00%	1.64%