FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) usd_per_policy	Sep. 30, 2019 USD ($) usd_per_policy
Liabilities
Fair Value	$ 658,788,000	$ 1,424,955,000
Level 3 | Other asset-backed securities
Assets:
Fair value	421,458,000	415,950,000
Level 3 | Corporate securities
Assets:
Fair value	631,068,000	1,325,112,000
Level 3 | Embedded derivative - GLWB
Assets:
Fair value		217,757,000
Liabilities
Fair Value	43,307,000
Level 3 | Embedded derivative - FIA
Liabilities
Fair Value	217,288,000	308,861,000
Level 3 | Embedded derivative - IUL
Liabilities
Fair Value	$ 90,231,000	$ 141,793,000
Level 3 | Discounted cash flow | Corporate securities | Minimum
Liabilities
Debt securities,measurement input	0.0084	0.0089
Level 3 | Discounted cash flow | Corporate securities | Maximum
Liabilities
Debt securities,measurement input	0.0300	0.0393
Level 3 | Discounted cash flow | Corporate securities | Weighted Average
Liabilities
Debt securities,measurement input	0.0184	0.0173
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input | usd_per_policy	145	195
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Minimum
Liabilities
Debt securities, measurement input, value	$ 85.75	$ 95.39
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Maximum
Liabilities
Debt securities, measurement input, value	99.99	97
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Weighted Average
Liabilities
Debt securities, measurement input, value	$ 95.36	$ 96.48
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities,measurement input	0.0002	0.0001
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities,measurement input	0.0125	0.0132
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities,measurement input	0.0064	0.0044
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities,measurement input	0.1096	0.0996
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities,measurement input	0.1311	0.1261
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities,measurement input	0.1203	0.1162
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	1
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	1.56
Utilization | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB
Liabilities
Embedded derivative, measurement input	0.99
Policies that have a one-time over-utilization rate of a specified amount (as a percent)	10.00%
Specified level of one-time over-utilization (as a percent)	400.00%
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities
Embedded derivative, measurement input	0.0021	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities
Embedded derivative, measurement input	0.0116	0.0097
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.0021	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.0116	0.0097
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.0021	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	0.0116	0.0097
Withdrawal rate prior to age 70 | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	0.015	0.004
Withdrawal rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	1	0.012
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.010	0.005
Lapse | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.300	0.50
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	0.10	0.10