FAIR VALUE OF FINANCIAL INSTRUMENTS - Valuation of Level 3 Financial Instruments (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Liabilities:
Fair Value	$ 966,495,000	$ 748,821,000
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%
Equity securities
Assets:
Fair Value	$ 66,200,000
Level 3 | Other asset-backed securities
Assets:
Fair Value	553,005,000	553,308,000
Level 3 | Corporate securities
Assets:
Fair Value	615,576,000	638,279,000
Level 3 | Embedded derivative - GLWB
Liabilities:
Fair Value	131,276,000	115,370,000
Level 3 | Embedded derivative - FIA
Liabilities:
Fair Value	197,378,000	147,368,000
Level 3 | Embedded derivative - IUL
Liabilities:
Fair Value	69,081,000	46,051,000
Level 3 | Liquidation Technique | Other asset-backed securities | Minimum
Unobservable input
Liquidation value	90	88
Level 3 | Liquidation Technique | Other asset-backed securities | Maximum
Unobservable input
Liquidation value	97	97.25
Level 3 | Liquidation Technique | Other asset-backed securities | Weighted average
Unobservable input
Liquidation value	$ 95.02	$ 95.04
Level 3 | Discounted cash flow | Other asset-backed securities | Minimum
Unobservable input
Liquidity premium (as a percent)	0.51%
Paydown rate (as a percent)	10.75%
Level 3 | Discounted cash flow | Other asset-backed securities | Maximum
Unobservable input
Liquidity premium (as a percent)	1.29%
Paydown rate (as a percent)	12.75%
Level 3 | Discounted cash flow | Other asset-backed securities | Weighted average
Unobservable input
Liquidity premium (as a percent)	0.96%
Paydown rate (as a percent)	11.34%
Level 3 | Discounted cash flow | Corporate securities | Minimum
Unobservable input
Spread over treasury (as a percent)	0.35%	0.31%
Level 3 | Discounted cash flow | Corporate securities | Maximum
Unobservable input
Spread over treasury (as a percent)	4.35%	4.50%
Level 3 | Discounted cash flow | Corporate securities | Weighted average
Unobservable input
Spread over treasury (as a percent)	1.57%	2.04%
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)	0.30%	0.30%
Nonperformance risk (as a percent)	0.12%	0.18%
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)	15.00%	15.00%
Nonperformance risk (as a percent)	0.86%	1.09%
Level 3 | Actuarial cash flow model | Embedded derivative - FIA
Unobservable input
Expenses per Policy	$ 146	$ 126
Withdrawal rate prior to age 70 (as a percent)	1.50%	1.00%
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%	2.00%
Nonperformance risk (as a percent)	0.12%	0.18%
Asset earned rate (as a percent)	4.08%	4.08%
Level 3 | Actuarial cash flow model | Embedded derivative - FIA | Maximum
Unobservable input
Lapse rate (as a percent)	30.00%	40.00%
Nonperformance risk (as a percent)	0.86%	1.09%
Asset earned rate (as a percent)	4.66%	4.66%
Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Minimum
Unobservable input
Mortality (as a percent)	34.00%	38.00%
Lapse rate (as a percent)	0.50%	0.50%
Nonperformance risk (as a percent)	0.12%	0.18%
Level 3 | Actuarial cash flow model | Embedded derivative - IUL | Maximum
Unobservable input
Mortality (as a percent)	152.00%	153.00%
Lapse rate (as a percent)	10.00%	10.00%
Nonperformance risk (as a percent)	0.86%	1.09%