OFFSETTING OF ASSETS AND LIABILITIES - Offsetting of Derivative Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting of Derivative Assets
Gross Amounts of Recognized Assets	$ 468,871	$ 417,769
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	468,871	417,769
Gross Amounts Not Offset in the Statement of Financial Position, financial instruments	242,105	171,384
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Received	108,830	100,890
Net Amount	117,936	145,495
Derivatives not subject to a master netting arrangement or similar arrangement	160,666	115,664
Gross Amounts of Recognized Assets, Including not subject to master netting arrangement	629,537	533,433
Net Amounts of Assets Presented in the Statement of Financial Position, Including not subject to master netting arrangement	629,537	533,433
Net Amount, Including not subject to master netting arrangement	278,602	261,159
Free-Standing derivatives
Offsetting of Derivative Assets
Gross Amounts of Recognized Assets	468,871	417,769
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Assets Presented in the Statement of Financial Position	468,871	417,769
Gross Amounts Not Offset in the Statement of Financial Position, financial instruments	242,105	171,384
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Received	108,830	100,890
Net Amount	117,936	145,495
Embedded derivative - Modco reinsurance treaties
Offsetting of Derivative Assets
Derivatives not subject to a master netting arrangement or similar arrangement	1,009	2,573
Embedded derivative - GLWB
Offsetting of Derivative Assets
Derivatives not subject to a master netting arrangement or similar arrangement	67,879	64,064
Derivatives with PLC
Offsetting of Derivative Assets
Derivatives not subject to a master netting arrangement or similar arrangement	91,578	48,878
Other
Offsetting of Derivative Assets
Derivatives not subject to a master netting arrangement or similar arrangement	$ 200	$ 149