SECURED FINANCING - Summary of Gross Amounts, Amounts Offset and Net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Gross amounts -reverse repurchase agreements	$ 2,123,449	$ 650,040
Amounts offset - reverse repurchase agreement	(1,600,000)	0
Reverse repurchase agreements	523,449	650,040	[1]
Gross amounts -repurchase agreement	90,154,170	81,115,874
Amounts offset -repurchase agreement	(1,600,000)	0
Repurchase agreements outstanding	$ 88,554,170	$ 81,115,874	[1]

[1] (1) Derived from the audited consolidated financial statements at December 31, 2018.