FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) usd_per_policy	Dec. 31, 2018 USD ($) usd_per_policy
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	$ 1,547,178,000	$ 658,788,000
Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	76,800,000	39,700,000
Financial instruments with book value approximating fair value	73,000,000.0	63,400,000
Commercial mortgage-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	10,029,000
Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	421,219,000	421,458,000
Financial instruments that are valued using broker quotes	65,400,000	26,200,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	1,373,714,000	631,068,000
Financial instruments that are valued using broker quotes	11,400,000	13,500,000
Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	186,038,000	43,307,000
Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	336,826,000	217,288,000
Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Derivative liability	$ 151,765,000	$ 90,231,000
Discounted cash flow | Commercial mortgage-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.025
Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input | usd_per_policy	195	145
Minimum | Discounted cash flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0000	0.0084
Maximum | Discounted cash flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0403	0.0300
Weighted Average | Discounted cash flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0160	0.0184
Liquidation value | Minimum | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	$ 95.39	$ 85.75
Liquidation value | Maximum | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	99.99	99.99
Liquidation value | Weighted Average | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input, value	$ 97.95	$ 95.36
Liquidity premium | Minimum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0034	0.0002
Liquidity premium | Maximum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0228	0.0125
Liquidity premium | Weighted Average | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0144	0.0064
Paydown rate | Minimum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.0899	0.1096
Paydown rate | Maximum | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.1245	0.1311
Paydown rate | Weighted Average | Discounted cash flow | Level 3
Assets and liabilities measured at fair value on a recurring basis
Debt securities, measurement input	0.1128	0.1203
Mortality | Minimum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.87	0.87
Mortality | Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.87	0.87
Mortality | Minimum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
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 | Embedded derivative—IUL | Level 3
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, duration scale | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.94
Mortality | Maximum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	1	1
Mortality | Maximum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	1	1
Mortality | Maximum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		5.77
Mortality | Maximum | Actuarial cash flow model, vbt primary tables | Embedded derivative—IUL | Level 3
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, duration scale | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	2.48
Utilization | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.99	0.99
Policies with one time over utilization rate of specified amount as a percent	10.00%	10.00%
Specified one time over-utilization rate (as a percent)	400.00%	400.00%
Nonperformance risk | Minimum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0012	0.0021
Nonperformance risk | Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0012	0.0021
Nonperformance risk | Minimum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0021	0.0021
Nonperformance risk | Maximum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0082	0.0116
Nonperformance risk | Maximum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0082	0.0116
Nonperformance risk | Maximum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.0082	0.0116
Withdrawal rate prior to age 70 | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		0.015
Withdrawal rate prior to age 70 | Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
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 | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.012
Withdrawal rate | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input		1
Lapse | Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.005	0.010
Lapse | Minimum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.005	0.005
Lapse | Maximum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.500	0.300
Lapse | Maximum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Embedded derivative liability, measurement input	0.10	0.100