RESTRICTED CASH AND CASH EQUIVALENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Total restricted cash and cash equivalents	$ 1,503,813	$ 1,202,651
Cash held by prime brokers relating to securities sold, not yet purchased
Cash and Cash Equivalents [Line Items]
Total restricted cash and cash equivalents	912,796	859,901
Cash collateral relating to letters of credit issued
Cash and Cash Equivalents [Line Items]
Total restricted cash and cash equivalents	173,748	234,348
Cash held as collateral in trust accounts
Cash and Cash Equivalents [Line Items]
Total restricted cash and cash equivalents	377,932	86,351
Cash and cash equivalents held by swap counterparties
Cash and Cash Equivalents [Line Items]
Total restricted cash and cash equivalents	$ 39,337	$ 22,051