FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Liabilities:
Fair Value	$ 759,264,000	$ 1,007,572,000
Embedded derivative - GLWB | Minimum
Unobservable input
Mortality (as a percent)	91.10%
Embedded derivative - GLWB | Maximum
Unobservable input
Mortality (as a percent)	106.60%
Embedded derivative - FIA | Minimum
Unobservable input
Mortality (as a percent)	46.00%
Embedded derivative - FIA | Maximum
Unobservable input
Mortality (as a percent)	113.00%
Embedded derivative - IUL | Minimum
Unobservable input
Mortality (as a percent)	34.00%
Embedded derivative - IUL | Maximum
Unobservable input
Mortality (as a percent)	152.00%
Level 3 | Other asset-backed securities
Assets:
Fair value	$ 515,379,000	504,228,000
Level 3 | Corporate securities
Assets:
Fair value	637,479,000	617,770,000
Level 3 | Embedded derivative - GLWB
Liabilities:
Fair Value	19,913,000	111,760,000
Level 3 | Embedded derivative - FIA
Liabilities:
Fair Value	238,569,000	218,676,000
Level 3 | Embedded derivative - IUL
Liabilities:
Fair Value	83,166,000	80,212,000
Level 3 | Liquidation Technique | Other asset-backed securities | Minimum
Unobservable input
Liquidation value	90	90
Level 3 | Liquidation Technique | Other asset-backed securities | Maximum
Unobservable input
Liquidation value	97	97
Level 3 | Liquidation Technique | Other asset-backed securities | Weighted average
Unobservable input
Liquidation value	$ 94.91	$ 94.91
Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Unobservable input
Liquidity premium (as a percent)	0.06%	0.06%
Paydown rate (as a percent)	11.18%	11.31%
Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Unobservable input
Liquidity premium (as a percent)	0.90%	1.17%
Paydown rate (as a percent)	12.98%	11.97%
Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Unobservable input
Liquidity premium (as a percent)	0.38%	0.75%
Paydown rate (as a percent)	12.30%	11.54%
Level 3 | Discounted cash flow | Corporate securities | Minimum
Unobservable input
Spread over treasury (as a percent)	0.83%	0.81%
Level 3 | Discounted cash flow | Corporate securities | Maximum
Unobservable input
Spread over treasury (as a percent)	2.78%	3.95%
Level 3 | Discounted cash flow | Corporate securities | Weighted average
Unobservable input
Spread over treasury (as a percent)	1.53%	1.06%
Level 3 | Actuarial cash flow model | Embedded derivative - GLWB
Unobservable input
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%
Specified level of one-time over-utilization (as a percent)	400.00%	400.00%
Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Minimum
Unobservable input
Mortality (as a percent)	91.10%	91.10%
Lapse rate (as a percent)		1.00%
Nonperformance risk (as a percent)	0.13%	0.11%
Level 3 | Actuarial cash flow model | Embedded derivative - GLWB | Maximum
Unobservable input
Mortality (as a percent)	106.60%	106.60%
Lapse rate (as a percent)		30.00%
Nonperformance risk (as a percent)	0.93%	0.79%
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Unobservable input
Expenses per policy	$ 146	$ 146
Withdrawal rate prior to age 70 (as a percent)	1.50%	1.50%
Withdrawal rate for ages 70 or more (as a percent)	100.00%	100.00%
Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Minimum
Unobservable input
Lapse rate (as a percent)	1.00%	1.00%
Nonperformance risk (as a percent)	0.13%	0.11%
Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Unobservable input
Lapse rate (as a percent)	30.00%	30.00%
Nonperformance risk (as a percent)	0.93%	0.79%
Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Unobservable input
Mortality (as a percent)	34.00%	34.00%
Lapse rate (as a percent)	0.50%	0.50%
Nonperformance risk (as a percent)	0.13%	0.11%
Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Unobservable input
Mortality (as a percent)	152.00%	152.00%
Lapse rate (as a percent)	10.00%	10.00%
Nonperformance risk (as a percent)	0.93%	0.79%