FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) usd_per_policy	Dec. 31, 2018 USD ($) usd_per_policy
Liabilities
Fair Value	$ 1,155,269,000	$ 658,788,000
Level 3 | Other asset-backed securities
Assets:
Fair value	418,178,000	421,458,000
Level 3 | Corporate securities
Assets:
Fair value	1,328,490,000	631,068,000
Level 3 | Embedded derivative - GLWB
Assets:
Fair value	146,556,000
Liabilities
Fair Value		43,307,000
Level 3 | Embedded derivative - FIA
Liabilities
Fair Value	300,364,000	217,288,000
Level 3 | Embedded derivative - IUL
Liabilities
Fair Value	$ 136,660,000	$ 90,231,000
Level 3 | Discounted cash flow | Corporate securities | Minimum
Liabilities
Debt securities,measurement input	0.0087	0.0084
Level 3 | Discounted cash flow | Corporate securities | Maximum
Liabilities
Debt securities,measurement input	0.0410	0.0300
Level 3 | Discounted cash flow | Corporate securities | Weighted Average
Liabilities
Debt securities,measurement input	0.0175	0.0184
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input | usd_per_policy	145	145
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Minimum
Liabilities
Debt securities, measurement input, value	$ 95.39	$ 85.75
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Maximum
Liabilities
Debt securities, measurement input, value	97.00	99.99
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Weighted Average
Liabilities
Debt securities, measurement input, value	$ 96.48	$ 95.36
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities,measurement input	0.0011	0.0002
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities,measurement input	0.0150	0.0125
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities,measurement input	0.0073	0.0064
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities,measurement input	0.1084	0.1096
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities,measurement input	0.1288	0.1311
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities,measurement input	0.1198	0.1203
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities
Embedded derivative, measurement input	0.87	0.87
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities
Embedded derivative, measurement input	1.00	1.000
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.37	0.37
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	5.77	5.77
Utilization | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB
Liabilities
Embedded derivative, measurement input	0.99	0.99
Policies that have a one-time over-utilization rate of a specified amount (as a percent)	10.00%	10.00%
Specified level of one-time over-utilization (as a percent)	400.00%	400.00%
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities
Embedded derivative, measurement input	0.0017	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0116
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.0017	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0116
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.0017	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0116
Withdrawal rate prior to age 70 | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	0.015	0.015
Withdrawal rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	1	1.00
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.010	0.010
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.300	0.300
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.005	0.005
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	0.100	0.100