FAIR VALUE OF FINANCIAL INSTRUMENTS - Determination of Fair Values Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) primary_source	Dec. 31, 2017 USD ($) quote	Dec. 31, 2017 USD ($) input_method	Dec. 31, 2016 USD ($)
Determination of fair values
Transferred out of Level 3			$ 28,900,000				$ 221,500,000
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		92.70%
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%
Fair value, liabilities	$ 1,007,572,000	$ 1,007,572,000	1,007,572,000	$ 1,007,572,000	$ 1,007,572,000	$ 1,007,572,000	748,821,000
Transfers from Level 2 to Level 1	0	0	0	0	0	0	12,200,000
Transfers from Level 1 to Level 2	0	0	0	0	0	0	100,000
Level 3
Determination of fair values
Financial instruments that are valued using broker quotes	50,400,000	50,400,000	50,400,000	50,400,000	50,400,000	50,400,000	128,200,000
Financial instruments with book value approximating fair value	65,700,000	65,700,000	65,700,000	65,700,000	65,700,000	65,700,000	$ 65,900,000
Asset-backed securities | Level 2
Determination of fair values
Fair value, assets	5,500,000,000	5,500,000,000	5,500,000,000	5,500,000,000	5,500,000,000	5,500,000,000
Asset-backed securities | Level 3
Determination of fair values
Fair value, assets	539,600,000	$ 539,600,000	539,600,000	539,600,000	539,600,000	539,600,000
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, assets	504,228,000	$ 504,228,000	504,228,000	504,228,000	504,228,000	504,228,000	$ 553,308,000
Financial instruments that are valued using broker quotes	35,400,000	35,400,000	35,400,000	35,400,000	35,400,000	35,400,000	93,900,000
Other asset-backed securities | Level 3 | Available-for-sale securities
Determination of fair values
Fair value, assets	504,400,000	$ 504,400,000	504,400,000	504,400,000	504,400,000	504,400,000
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, assets	35,200,000	$ 35,200,000	35,200,000	35,200,000	35,200,000	35,200,000
Corporate securities | Level 3
Determination of fair values
Fair value, assets	617,770,000	617,770,000	617,770,000	617,770,000	617,770,000	617,770,000	638,279,000
Financial instruments that are valued using broker quotes	14,600,000	14,600,000	14,600,000	14,600,000	14,600,000	14,600,000	31,300,000
Corporate Bonds, U.S. Government-Related Securities, and Other Government Related Securities | Level 2
Determination of fair values
Fair value, assets	33,500,000,000	33,500,000,000	33,500,000,000	33,500,000,000	33,500,000,000	33,500,000,000
Corporate Bonds, U.S. Government-Related Securities, and Other Government Related Securities | Level 3
Determination of fair values
Fair value, assets	$ 632,300,000	632,300,000	632,300,000	632,300,000	632,300,000	632,300,000
Embedded derivative - GLWB
Determination of fair values
Discount rate curve, base rate	LIBOR
Fair value, liabilities	$ 2,400,000,000	$ 2,400,000,000	2,400,000,000	2,400,000,000	2,400,000,000	2,400,000,000
Unrealized gain (loss) of securities			226,600,000
Embedded derivative - GLWB | Minimum
Determination of fair values
Mortality (as a percent)		91.10%
Embedded derivative - GLWB | Maximum
Determination of fair values
Mortality (as a percent)		106.60%
Embedded derivative - GLWB | Level 3
Determination of fair values
Fair value, assets							$ 115,370,000
Fair value, liabilities	111,760,000	$ 111,760,000	111,760,000	111,760,000	111,760,000	111,760,000
Embedded derivative - GLWB | Level 3 | Actuarial cash flow model | Minimum
Determination of fair values
Mortality (as a percent)		91.10%					91.10%
Embedded derivative - GLWB | Level 3 | Actuarial cash flow model | Maximum
Determination of fair values
Mortality (as a percent)		1.00%					106.60%
Equity securities | Level 2 and Level 3
Determination of fair values
Fair value, assets	66,100,000	$ 66,100,000	66,100,000	66,100,000	66,100,000	66,100,000
Federal home loan bank stock	65,500,000	65,500,000	65,500,000	65,500,000	65,500,000	65,500,000
Equity securities | Level 3
Determination of fair values
Financial instruments that are valued using broker quotes	$ 400,000	400,000	400,000	400,000	400,000	400,000	$ 3,100,000
Embedded derivative—FIA
Determination of fair values
Discount rate curve, base rate	LIBOR
Embedded derivative—FIA | Level 3
Determination of fair values
Fair value, liabilities	$ 218,676,000	$ 218,676,000	218,676,000	218,676,000	218,676,000	218,676,000	147,368,000
Embedded derivative—FIA | Level 3 | Annuity account | Actuarial cash flow model | Minimum
Determination of fair values
Mortality (as a percent)		46.00%
Embedded derivative—FIA | Level 3 | Annuity account | Actuarial cash flow model | Maximum
Determination of fair values
Mortality (as a percent)		113.00%
Embedded derivative—IUL | Annuity account
Determination of fair values
Discount rate curve, base rate	LIBOR
Embedded derivative—IUL | Level 3
Determination of fair values
Fair value, liabilities	$ 80,212,000	$ 80,212,000	80,212,000	80,212,000	80,212,000	80,212,000	$ 46,051,000
Embedded derivative—IUL | Level 3 | Actuarial cash flow model | Minimum
Determination of fair values
Mortality (as a percent)		34.00%					38.00%
Embedded derivative—IUL | Level 3 | Actuarial cash flow model | Maximum
Determination of fair values
Mortality (as a percent)		152.00%					153.00%
Measured at fair value on a recurring basis | Level 2
Determination of fair values
Fair value, assets	39,499,120,000	$ 39,499,120,000	39,499,120,000	39,499,120,000	39,499,120,000	39,499,120,000	$ 36,110,555,000
Annuity account balances	0	0	0	0	0	0	0
Measured at fair value on a recurring basis | Level 3
Determination of fair values
Fair value, assets	1,374,044,000	1,374,044,000	1,374,044,000	1,374,044,000	1,374,044,000	1,374,044,000	1,510,043,000
Annuity account balances	$ 83,472,000	$ 83,472,000	$ 83,472,000	$ 83,472,000	$ 83,472,000	$ 83,472,000	$ 87,616,000