FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets and liabilities measured at fair value on a recurring basis
Transfers from Level 1 to Level 2	$ 0	$ 100,000
Fair value, liabilities	1,007,572,000	748,821,000
Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	504,228,000	553,308,000
Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets	617,770,000	638,279,000
Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Fair value, liabilities	2,400,000,000
Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, assets		115,370,000
Fair value, liabilities	111,760,000
Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, liabilities	218,676,000	147,368,000
Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value, liabilities	$ 80,212,000	$ 46,051,000
Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Specified one-time over-utilization rate (as a percent)	400.00%
Utilization (as a percent)	99.00%	99.00%
Policies that have a one-time over-utilization rate of a specified amount (as a percent)	10.00%	10.00%
Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Expenses per policy	$ 146	$ 126
Withdrawal rate prior to age 70 (as a percent)	1.50%
Withdrawal rate for ages 70 or more (as a percent)	100.00%
Minimum
Assets and liabilities measured at fair value on a recurring basis
Paydown rate (as a percent)	11.31%
Minimum | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	91.10%
Minimum | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidation value	$ 90	$ 88
Minimum | Discounted Cash Flow | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidity Premium	0.06%
Minimum | Discounted Cash Flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Spread over treasury (as a percent)	0.81%	0.31%
Minimum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	91.10%	91.10%
Fair value inputs, lapse rate	1.00%	0.30%
Nonperformance risk (as a percent)	0.11%	0.18%
Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value inputs, lapse rate	1.00%	2.00%
Nonperformance risk (as a percent)	0.11%	0.18%
Asset earned rate (as a percent)		4.08%
Withdrawal rate (as a percent)		1.00%
Minimum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	34.00%	38.00%
Fair value inputs, lapse rate	0.50%	0.50%
Nonperformance risk (as a percent)	0.11%	0.18%
Maximum
Assets and liabilities measured at fair value on a recurring basis
Paydown rate (as a percent)	11.97%
Maximum | Embedded derivative - GLWB
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	106.60%
Maximum | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidation value	$ 97.00	$ 97.25
Maximum | Discounted Cash Flow | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidity Premium	1.17%
Maximum | Discounted Cash Flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Spread over treasury (as a percent)	3.95%	4.50%
Maximum | Actuarial cash flow model | Embedded derivative - GLWB | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	1.00%	106.60%
Fair value inputs, lapse rate	30.00%	15.00%
Nonperformance risk (as a percent)	0.79%	1.09%
Maximum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value inputs, lapse rate	30.00%	40.00%
Nonperformance risk (as a percent)	0.79%	1.09%
Asset earned rate (as a percent)		4.66%
Withdrawal rate (as a percent)		100.00%
Maximum | Actuarial cash flow model | Embedded derivative—IUL | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	152.00%	153.00%
Fair value inputs, lapse rate	10.00%	10.00%
Nonperformance risk (as a percent)	0.79%	1.09%
Weighted Average
Assets and liabilities measured at fair value on a recurring basis
Paydown rate (as a percent)	11.54%
Weighted Average | Liquidation technique | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidation value	$ 94.91	$ 95.04
Weighted Average | Discounted Cash Flow | Other asset-backed securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Liquidity Premium	0.75%
Weighted Average | Discounted Cash Flow | Corporate securities | Level 3
Assets and liabilities measured at fair value on a recurring basis
Spread over treasury (as a percent)	1.06%	2.04%
Annuity account | Minimum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	46.00%
Annuity account | Maximum | Actuarial cash flow model | Embedded derivative—FIA | Level 3
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	113.00%