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, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Gross amounts -reverse repurchase agreements	$ 3,375,459	$ 1,250,000
Amounts offset - reverse repurchase agreement	(3,175,000)	(1,250,000)
Netted amounts -reverse repurchase	200,459	0	[1]
Gross amounts -repurchase agreement	81,190,431	78,946,343
Amounts offset -repurchase agreement	(3,175,000)	(1,250,000)
Repurchase agreements	$ 78,015,431	$ 77,696,343	[1]

[1] Derived from the audited consolidated financial statements at December 31, 2017.