OFFSETTING OF ASSETS AND LIABILITIES - Schedule of Derivative Instruments by Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	$ 786,972	$ 905,973
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	786,972	905,973
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	194,876	242,105
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	1,727	4,577
Net Amount	590,369	659,291
Repurchase agreements
Gross Amounts of Recognized Liabilities	665,000	885,000
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	665,000	885,000
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	665,000	885,000
Total Liabilities
Gross Amounts of Recognized Liabilities	1,451,972	1,790,973
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	1,451,972	1,790,973
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	194,876	242,105
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	1,727	4,577
Net Amount	1,255,369	1,544,291
Free-Standing derivatives
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Subject to master netting arrangement or similar arrangement	196,603	246,682
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position, Subject to master netting arrangement or similar arrangement	196,603	246,682
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	194,876	242,105
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	1,727	4,577
Net Amount, Subject to master netting arrangement or similar arrangement	0	0
Derivatives, Subject to Master Netting Arrangement or Similar Arrangement
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Subject to master netting arrangement or similar arrangement	196,603	246,682
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position, Subject to master netting arrangement or similar arrangement	196,603	246,682
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	194,876	242,105
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	1,727	4,577
Net Amount, Subject to master netting arrangement or similar arrangement	0	0
Embedded derivative - Modco reinsurance treaties
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	126,090	215,247
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	126,090	215,247
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	126,090	215,247
Funds withheld derivative
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	101,773	61,729
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	101,773	61,729
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	101,773	61,729
Embedded derivative - GLWB
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	66,816	83,427
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	66,816	83,427
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	66,816	83,427
Embedded derivative - FIA
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	218,340	218,676
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	218,340	218,676
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	218,340	218,676
Embedded derivative - IUL
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	77,350	80,212
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	77,350	80,212
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	77,350	80,212
Total derivatives, not subject to a master netting arrangement or similar arrangement
Offsetting of Liabilities
Gross Amounts of Recognized Liabilities, Not subject to a master netting arrangement or similar arrangement	590,369	659,291
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	590,369	659,291
Derivative Liability, Not Subject to Master Netting Arrangement Deduction	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Paid	0	0
Net Amount	$ 590,369	$ 659,291