OFFSETTING OF ASSETS AND LIABILITIES - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	$ 1,007,572	$ 748,821
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	1,007,572	748,821
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 Paid	4,577	5,595
Net Amount	760,890	571,842
Repurchase agreements
Gross Amounts of Recognized Liabilities	885,000	797,721
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	885,000	797,721
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	885,000	797,721
Total Liabilities
Gross Amounts of Recognized Liabilities	1,892,572	1,546,542
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	1,892,572	1,546,542
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, Cash Collateral Paid	4,577	5,595
Net Amount	1,645,890	1,369,563
Derivatives not subject to a master netting arrangement or similar arrangement
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	246,682	176,979
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	246,682	176,979
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 Paid	4,577	5,595
Net Amount	0	0
Free-Standing derivatives
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	246,682	176,979
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	246,682	176,979
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 Paid	4,577	5,595
Net Amount	0	0
Total derivatives, not subject to a master netting arrangement or similar arrangement
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	760,890	571,842
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	760,890	571,842
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Paid	0	0
Net Amount	760,890	571,842
Embedded derivative - Modco reinsurance treaties
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	215,247	141,301
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	215,247	141,301
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Paid	0	0
Net Amount	215,247	141,301
Embedded derivative - GLWB
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	246,755	237,122
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	246,755	237,122
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Paid	0	0
Net Amount	246,755	237,122
Embedded derivative—FIA
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	218,676	147,368
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	218,676	147,368
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Paid	0	0
Net Amount	218,676	147,368
Embedded derivative—IUL
Offsetting of Derivative Liabilities:
Gross Amounts of Recognized Liabilities	80,212	46,051
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statement of Financial Position	80,212	46,051
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Collateral Paid	0	0
Net Amount	$ 80,212	$ 46,051