BORROWINGS - Significant Counterparties of Repurchase Agreements (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 1,177,060,000	$ 1,234,522,000
Percent of total amount outstanding	100.00%	100.00%
Weighted days to maturity	12 days	13 days
Market Value of collateral held	$ 1,237,603,856	$ 1,295,225,428
North America
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 895,289,000	$ 939,438,000
Percent of total amount outstanding	76.06%	76.10%
Weighted days to maturity	11 days	13 days
Market Value of collateral held	$ 941,208,763	$ 985,672,703
Asia
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 278,992,000	$ 292,529,000
Percent of total amount outstanding	23.70%	23.70%
Weighted days to maturity	16 days	14 days
Market Value of collateral held	$ 292,242,600	$ 305,152,946
Europe
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 2,779,000	$ 2,555,000
Percent of total amount outstanding	0.24%	0.20%
Weighted days to maturity	80 days	78 days
Market Value of collateral held	$ 4,152,493	$ 4,399,779