FAIR VALUE OF FINANCIAL INSTRUMENTS - Determination of Fair Values Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) primary_source	Dec. 31, 2018 USD ($) quote	Dec. 31, 2018 USD ($) input_method	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Determination of fair values
Number of primary sources of information used for determining fair value | primary_source				1
Total number of primary sources of information available for determining fair value				3		3
Minimum percentage of the Company's fixed maturity securities priced by third party pricing services		93.60%
Number of independent non-binding broker quotes obtained per security | quote					1
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations		100.00%
Transfers into Level 3			$ 39,700,000				$ 0
Transfers from Level 3 to Level 2			85,700,000				28,900,000
Transfers from Level 2 to Level 1	$ 0	$ 0	0	$ 0	$ 0	$ 0	0
Transfers from Level 1 to Level 2	0	0	0	0	0	0	0
Level 3
Determination of fair values
Annuity account balances	706,061,000	706,061,000	706,061,000	706,061,000	706,061,000	706,061,000	844,362,000	$ 659,459,000
Financial instruments that are valued using broker quotes	40,400,000	40,400,000	40,400,000	40,400,000	40,400,000	40,400,000	50,400,000
Financial instruments with book value approximating fair value	63,600,000	63,600,000	63,600,000	63,600,000	63,600,000	63,600,000	$ 65,700,000
Asset-backed securities | Level 2
Determination of fair values
Fair value	7,400,000,000	7,400,000,000	7,400,000,000	7,400,000,000	7,400,000,000	7,400,000,000
Asset-backed securities | Level 3
Determination of fair values
Fair value	447,700,000	$ 447,700,000	447,700,000	447,700,000	447,700,000	447,700,000
Other asset-backed securities
Determination of fair values
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (at least)		97.00%					97.00%
Other asset-backed securities | Level 3
Determination of fair values
Fair value	421,458,000	$ 421,458,000	421,458,000	421,458,000	421,458,000	421,458,000	$ 504,228,000
Financial instruments that are valued using broker quotes	26,200,000	26,200,000	26,200,000	26,200,000	26,200,000	26,200,000	35,400,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Determination of fair values
Fair value	421,600,000	$ 421,600,000	421,600,000	421,600,000	421,600,000	421,600,000
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (at least)		97.00%
Other asset-backed securities | Level 3 | Trading securities
Determination of fair values
Fair value	26,100,000	$ 26,100,000	26,100,000	26,100,000	26,100,000	26,100,000
Corporate Bonds and Securities | Level 2
Determination of fair values
Fair value	42,300,000,000	42,300,000,000	42,300,000,000	42,300,000,000	42,300,000,000	42,300,000,000
Corporate Bonds and Securities | Level 3
Determination of fair values
Fair value	644,500,000	644,500,000	644,500,000	644,500,000	644,500,000	644,500,000
Corporate securities | Level 3
Determination of fair values
Fair value	631,068,000	631,068,000	631,068,000	631,068,000	631,068,000	631,068,000	617,770,000
Financial instruments that are valued using broker quotes	13,500,000	13,500,000	13,500,000	13,500,000	13,500,000	13,500,000	14,600,000
Equity securities | Level 2 and Level 3
Determination of fair values
Fair value	64,400,000	64,400,000	64,400,000	64,400,000	64,400,000	64,400,000
Federal home loan bank stock	63,400,000	63,400,000	63,400,000	63,400,000	63,400,000	63,400,000
Equity securities | Level 3
Determination of fair values
Financial instruments that are valued using broker quotes	$ 700,000	$ 700,000	700,000	700,000	700,000	700,000	400,000
Embedded derivative - GLWB
Determination of fair values
Discount rate curve, base rate	LIBOR
Embedded derivative - GLWB | Minimum
Determination of fair values
Mortality (as a percent)		87.00%
Embedded derivative - GLWB | Maximum
Determination of fair values
Mortality (as a percent)		100.00%
Embedded derivative - GLWB | Level 3
Determination of fair values
Fair value							111,760,000
Embedded derivative—FIA
Determination of fair values
Discount rate curve, base rate	LIBOR
Embedded derivative—FIA | Level 3 | Actuarial cash flow model | Minimum
Determination of fair values
Mortality (as a percent)		87.00%
Embedded derivative—FIA | Level 3 | Actuarial cash flow model | Maximum
Determination of fair values
Mortality (as a percent)		100.00%
Embedded derivative—IUL
Determination of fair values
Discount rate curve, base rate	LIBOR
Embedded derivative—IUL | Level 3 | Actuarial cash flow model | Minimum
Determination of fair values
Mortality (as a percent)		37.00%
Embedded derivative—IUL | Level 3 | Actuarial cash flow model | Maximum
Determination of fair values
Mortality (as a percent)		577.00%
Embedded derivative - MCA
Determination of fair values
Fair value, liabilities	$ 2,300,000,000	$ 2,300,000,000	2,300,000,000	2,300,000,000	2,300,000,000	2,300,000,000
Embedded derivative - MCA | Trading securities
Determination of fair values
Fair value	25,800,000	25,800,000	25,800,000	25,800,000	25,800,000	25,800,000
VA | Level 3
Determination of fair values
Annuity account balances	$ 76,119,000	$ 76,119,000	$ 76,119,000	$ 76,119,000	$ 76,119,000	$ 76,119,000	$ 83,472,000	$ 87,616,000