FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	Dec. 31, 2020 USD ($) usd_per_policy	Dec. 31, 2019 USD ($) usd_per_policy
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	$ 2,681,451,000	$ 1,547,178,000
Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	115,600,000	76,800,000
Financial instruments with book value approximating fair value	89,900,000	73,000,000.0
Level 3 | Commercial mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	32,425,000	10,029,000
Level 3 | Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	434,676,000	421,219,000
Financial instruments that are valued using broker quotes	87,600,000	65,400,000
Level 3 | Corporate securities
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	1,431,818,000	1,373,714,000
Financial instruments that are valued using broker quotes	16,700,000	11,400,000
Level 3 | Equity securities
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	11,300,000
Level 3 | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	403,656,000	186,038,000
Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	573,216,000	336,826,000
Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	$ 201,331,000	$ 151,765,000
Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input | usd_per_policy	207	195
Minimum | Discounted cash flow | Level 3 | Commercial mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input		0.025
Minimum | Discounted cash flow | Level 3 | Corporate securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0000	0.0000
Maximum | Discounted cash flow | Level 3 | Corporate securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0475	0.0403
Weighted Average | Discounted cash flow | Level 3 | Corporate securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0189	0.0160
Spread over treasury | Minimum | Discounted cash flow | Level 3 | Commercial mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0278
Spread over treasury | Maximum | Discounted cash flow | Level 3 | Commercial mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0292
Spread over treasury | Weighted Average | Discounted cash flow | Level 3 | Commercial mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0287
Liquidation value | Minimum | Liquidation | Level 3 | Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	$ 95.00	$ 95.39
Liquidation value | Maximum | Liquidation | Level 3 | Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	97.00	99.99
Liquidation value | Weighted Average | Liquidation | Level 3 | Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	$ 96.19	$ 97.95
Liquidity premium | Minimum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0054	0.0034
Liquidity premium | Maximum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0230	0.0228
Liquidity premium | Weighted Average | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0163	0.0144
Paydown Rate | Minimum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0879	0.0899
Paydown Rate | Maximum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.1249	0.1245
Paydown Rate | Weighted Average | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.1139	0.1128
Mortality | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.88	0.87
Mortality | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.88	0.87
Mortality | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		0.37
Mortality | Minimum | Actuarial cash flow model, vbt primary tables | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.36
Mortality | Minimum | Actuarial cash flow model, vbt primary tables, duration scale | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.94	0.94
Mortality | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	1	1
Mortality | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	1	1
Mortality | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		1.56
Mortality | Maximum | Actuarial cash flow model, vbt primary tables | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	1.61
Mortality | Maximum | Actuarial cash flow model, vbt primary tables, duration scale | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	2.48	2.48
Nonperformance risk | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0019	0.0012
Nonperformance risk | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0019	0.0012
Nonperformance risk | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0019	0.0012
Nonperformance risk | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0081	0.0082
Nonperformance risk | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0081	0.0082
Nonperformance risk | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0081	0.0082
Withdrawal rate prior to age 70 | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		0.004
Withdrawal rate prior to age 70 | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.004
Withdrawal rate prior to age 70 | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.024
Withdrawal rate | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		0.012
Lapse | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.002	0.005
Lapse | Minimum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.00375	0.005
Lapse | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - FIA
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.500	0.500
Lapse | Maximum | Actuarial cash flow model | Level 3 | Embedded derivative - IUL
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.10	0.100