BORROWINGS - Maturities of Repurchase Agreements (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Repurchase Agreements [Line Items]
Repurchase agreements	$ 1,177,060,000	$ 1,234,522,000
Maturity Less Than or Equal to 30 Days
Repurchase Agreements [Line Items]
Repurchase agreements	1,174,281,000	1,175,407,000
Maturity 31 To 60 Days
Repurchase Agreements [Line Items]
Repurchase agreements	0	56,560,000
Maturity 61 To 90 Days
Repurchase Agreements [Line Items]
Repurchase agreements	$ 2,779,000	$ 2,555,000