FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) usd_per_policy	Dec. 31, 2018 USD ($) usd_per_policy
Liabilities:
Fair Value	$ 972,965,000	$ 755,461,000
Level 3 | Other asset-backed securities
Assets:
Fair value	419,915,000	421,458,000
Level 3 | Corporate securities
Assets:
Fair value	648,608,000	631,068,000
Level 3 | Embedded derivative - GLWB
Liabilities:
Fair Value	203,696,000	184,071,000
Level 3 | Embedded derivative - FIA
Liabilities:
Fair Value	264,430,000	217,288,000
Level 3 | Embedded derivative - IUL
Liabilities:
Fair Value	$ 112,814,000	$ 90,231,000
Level 3 | Discounted cash flow | Corporate securities | Minimum
Liabilities:
Debt securities, measurement input	0.0092	0.0084
Level 3 | Discounted cash flow | Corporate securities | Maximum
Liabilities:
Debt securities, measurement input	0.0387	0.0300
Level 3 | Discounted cash flow | Corporate securities | Weighted average
Liabilities:
Debt securities, measurement input	0.0165	0.0184
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities:
Embedded derivative, measurement input	145	145
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input, value	$ 95.39	$ 85.75
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input, value	99.99	99.99
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input, value	$ 98.00	$ 95.36
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input	0.0004	0.0002
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input	0.0127	0.0125
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input	0.0054	0.0064
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input	0.1090	0.1096
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input	0.1309	0.1311
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input	0.1197	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	1.00
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities:
Embedded derivative, measurement input		1.00
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities:
Embedded derivative, measurement input	0.37	0.0037
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities:
Embedded derivative, measurement input	5.77	0.0577
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
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	10.00%	10.00%
Specified level of one-time over-utilization	400.00%	400.00%
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities:
Embedded derivative, measurement input	0.0023	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities:
Embedded derivative, measurement input	0.0101	0.0116
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities:
Embedded derivative, measurement input	0.0023	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities:
Embedded derivative, measurement input	0.0101	0.0116
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities:
Embedded derivative, measurement input	0.0023	0.0021
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities:
Embedded derivative, measurement input	0.0101	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	1.000	1.00