FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	9 Months Ended
	Sep. 30, 2020 USD ($) usd_per_policy	Dec. 31, 2019 USD ($) usd_per_policy
Liabilities
Fair Value	$ 2,403,594,000	$ 1,547,178,000
Level 3 | Commercial mortgage-backed securities
Assets:
Fair value	10,685,000	10,029,000
Level 3 | Other asset-backed securities
Assets:
Fair value	434,496,000	421,219,000
Level 3 | Corporate securities
Assets:
Fair value	1,357,544,000	1,373,714,000
Level 3 | Embedded derivative - GLWB
Assets:
Fair value	518,246,000
Liabilities
Fair Value		186,038,000
Level 3 | Embedded derivative - FIA
Liabilities
Fair Value	531,814,000	336,826,000
Level 3 | Embedded derivative - IUL
Liabilities
Fair Value	$ 194,228,000	$ 151,765,000
Level 3 | Discounted cash flow | Commercial mortgage-backed securities
Liabilities
Debt securities, measurement input	0.0278	2.5
Level 3 | Discounted cash flow | Corporate securities | Minimum
Liabilities
Debt securities, measurement input	0.0060	0.0000
Level 3 | Discounted cash flow | Corporate securities | Maximum
Liabilities
Debt securities, measurement input	0.0475	0.0403
Level 3 | Discounted cash flow | Corporate securities | Weighted Average
Liabilities
Debt securities, measurement input	0.0224	0.0160
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input | usd_per_policy	207	195
Liquidation value | Level 3 | Liquidation | Other asset-backed securities | Minimum
Liabilities
Debt securities, measurement input, value	$ 95.75	$ 95.39
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	$ 96.40	$ 97.95
Liquidity premium | Level 3 | Discounted cash flow | Minimum
Liabilities
Debt securities, measurement input		0.0034
Liquidity premium | Level 3 | Discounted cash flow | Maximum
Liabilities
Debt securities, measurement input		0.0228
Liquidity premium | Level 3 | Discounted cash flow | Weighted Average
Liabilities
Debt securities, measurement input		0.0144
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities, measurement input	0.0051
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities, measurement input	0.0233
Liquidity premium | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities, measurement input	0.0161
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Liabilities
Debt securities, measurement input	0.0875	0.0899
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Liabilities
Debt securities, measurement input	0.1255	0.1245
Paydown rate | Level 3 | Discounted cash flow | Other asset-backed securities | Weighted Average
Liabilities
Debt securities, measurement input	0.1138	0.1128
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Liabilities
Embedded derivative, measurement input	0.88	0.87
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities
Embedded derivative, measurement input	1	1
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.88	0.87
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	1	1
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.37	0.37
Mortality rate | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	1.61	1.56
Mortality rate | Level 3 | Actuarial Cash Flow Model, Valuation Basic Report Primary Tables Based On Duration Scale | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.94	0.94
Mortality rate | Level 3 | Actuarial Cash Flow Model, Valuation Basic Report Primary Tables Based On Duration Scale | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	2.48	2.48
Utilization rate | 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	0.0022	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0082
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.0022	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0082
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.0022	0.0012
Nonperformance risk | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	0.0088	0.0082
Withdrawal rate prior to age 70 | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	0.004	0.004
Withdrawal rate for ages over 70 | Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Liabilities
Embedded derivative, measurement input	0.024	0.012
Lapse rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Liabilities
Embedded derivative, measurement input	0.002	0.005
Lapse rate | Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Liabilities
Embedded derivative, measurement input	0.500	0.500
Lapse rate | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Liabilities
Embedded derivative, measurement input	0.004	0.005
Lapse rate | Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Liabilities
Embedded derivative, measurement input	0.10	0.10