BORROWINGS - Significant Counterparties of Repurchase Agreements (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 1,237,661,000	$ 804,811,000
Percent of total amount outstanding	100.00%	100.00%
Weighted days to maturity	14 days	16 days
Market Value of collateral held	$ 1,309,061,643	$ 876,121,505
Wells Fargo Bank
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 214,930,000
Percent of total amount outstanding	17.37%
Weighted days to maturity	9 days
Market Value of collateral held	$ 225,340,847
Wells Fargo Securities
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding		$ 33,666,000
Percent of total amount outstanding		4.18%
Weighted days to maturity		8 days
Market Value of collateral held		$ 57,627,433
Other North America
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 755,260,000	$ 703,788,000
Percent of total amount outstanding	61.02%	87.45%
Weighted days to maturity	14 days	16 days
Market Value of collateral held	$ 802,125,608	$ 742,690,286
Asia
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 264,721,000	$ 62,733,000
Percent of total amount outstanding	21.39%	7.79%
Weighted days to maturity	17 days	14 days
Market Value of collateral held	$ 277,019,585	$ 66,198,478
Europe
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 2,750,000	$ 4,624,000
Percent of total amount outstanding	0.22%	0.58%
Weighted days to maturity	80 days	44 days
Market Value of collateral held	$ 4,575,603	$ 9,605,308