FAIR VALUE - Securities in Unrealized Loss Position (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Securities in an unrealized loss position, fair value [Abstract]
One year or greater	$ 2,342,577	$ 2,552,668
Less than one year	4,844,667	4,665,883
Fair Value, Total	7,187,244	7,218,551
Securities in an unrealized loss position, aggregate loss [Abstract]
One year or greater	34,142	26,701
Less than one year	54,005	22,752
Unrealized Losses, Total	$ 88,147	$ 49,453