FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017 USD ($) usd_per_policy	Sep. 30, 2018 USD ($)	Sep. 30, 2018 usd_per_policy
Liabilities:
Fair Value		$ 1,007,572,000	$ 835,447,000
Level 3 | Other asset-backed securities
Assets:
Fair value		504,228,000	502,020,000
Level 3 | Corporate securities
Assets:
Fair value		617,770,000	621,392,000
Level 3 | Embedded derivative - GLWB
Liabilities:
Fair Value		111,760,000	37,794,000
Level 3 | Embedded derivative - FIA
Liabilities:
Fair Value		218,676,000	256,912,000
Level 3 | Embedded derivative - IUL
Liabilities:
Fair Value		$ 80,212,000	95,739,000
Level 3 | Discounted cash flow | Corporate securities | Minimum
Liabilities:
Debt securities, measurement input	0.0090	0.0081
Level 3 | Discounted cash flow | Corporate securities | Maximum
Liabilities:
Debt securities, measurement input	0.0423	0.0395
Level 3 | Discounted cash flow | Corporate securities | Weighted average
Liabilities:
Debt securities, measurement input	0.0151	0.0106
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities:
Embedded derivative, measurement input	1.000	146		145
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input, value		$ 90	90
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input, value		97	97
Liquidation value | Level 3 | Liquidation Technique | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input, value		$ 94.91	$ 94.92
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input	0.0000	0.0006
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input	0.0140	0.0117
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input	0.0062	0.0075
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities:
Debt securities, measurement input	0.1109	0.1131
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities:
Debt securities, measurement input	0.1315	0.1197
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Liabilities:
Debt securities, measurement input	0.1238	0.1154
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities:
Embedded derivative, measurement input	0.87	0.911
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities:
Embedded derivative, measurement input	1.00	1.066
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities:
Embedded derivative, measurement input	0.37	0.34
Mortality | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities:
Embedded derivative, measurement input	5.77	1.52
Lapse | Minimum
Liabilities:
Embedded derivative, measurement input		0.010
Lapse | Maximum
Liabilities:
Embedded derivative, measurement input		0.300
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	0.0013	0.0011
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities:
Embedded derivative, measurement input	0.0088	0.0079
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities:
Embedded derivative, measurement input	0.0013	0.0011
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities:
Embedded derivative, measurement input	0.3000	0.0079
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities:
Embedded derivative, measurement input	0.0013	0.0011
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities:
Embedded derivative, measurement input	0.0088	0.0079
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		1.00