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Represents the amount of transfers of assets, measured on a recurring basis, from Level 2 to Level 1 during the period.

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Fair Value Assets Transfers from Level 1

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Represents the number of quotes obtained per security when using non-binding independent broker quotations to establish fair value.

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Represents the number of primary sources of information available for determining the fair value of fixed maturity, short-term and equity securities.

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Represents the number of primary sources of information considered for determining the fair value of fixed maturity, short-term and equity securities.

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Fair Value Determination, Percentage of Fixed Maturity Securities, Priced by Third Party Pricing Services, Minimum

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Represents the mortality rate, used as an input to measure the fair value.

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Represents the fair value of financial instruments that are valued using broker quotes.

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Represents the percentage of derivatives based upon notional values, excluding embedded derivatives that were priced using exchange prices or independent broker quotations.

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Represents the minimum percentage of the underlying collateral guaranteed by a third party.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Fair value of firm holdings in securities classified as other. Includes pledged and unpledged holdings.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Amount of accrued obligation to policyholders that relates to insured events for long-duration contracts.

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FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) quote primary_source input_method	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Assets and liabilities measured at fair value on a recurring basis
Number of sources of information considered \| primary_source	1
Number of sources of information \| input_method	3
Minimum percentage of the Company's fixed maturity securities priced by third party pricing services	93.50%
Number of 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%
Financial instruments with book value approximating fair value	$ 63,600,000		$ 63,600,000
Transfers into Level 3	36,000,000	$ 0
Securities transferred from level 3 to level 2	19,000,000	0
Transfers from Level 2 to Level 1	0	0
Transfers from Level 1	0	0
Level 3
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	872,504,000	702,501,000	706,061,000	$ 844,362,000
Financial instruments that are valued using broker quotes	72,700,000		40,400,000
Level 3 \| Annuity account
Assets and liabilities measured at fair value on a recurring basis
Level 3 fair value	74,613,000	$ 81,399,000	76,119,000	$ 83,472,000
Asset-backed securities \| Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	7,700,000,000
Asset-backed securities \| Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	$ 486,600,000
Other asset-backed securities
Assets and liabilities measured at fair value on a recurring basis
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum	97.00%
Other asset-backed securities \| Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	$ 419,915,000		421,458,000
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum	97.00%
Financial instruments that are valued using broker quotes	$ 66,700,000		26,200,000
Other asset-backed securities \| Level 3 \| Available-for-sale securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	420,100,000
Other asset-backed securities \| Level 3 \| Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	66,500,000
Corporate bonds and securities \| Level 2
Assets and liabilities measured at fair value on a recurring basis
Fair value	43,900,000,000
Corporate bonds and securities \| Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	653,900,000
Equity securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	64,400,000
Federal Home Loan Bank (FHLB) stock	63,400,000
Equity securities \| Level 3
Assets and liabilities measured at fair value on a recurring basis
Financial instruments that are valued using broker quotes	$ 800,000		700,000
Embedded derivative - GLWB \| Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	87.00%
Embedded derivative - GLWB \| Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	100.00%
Embedded derivative - FIA \| Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	87.00%
Embedded derivative - FIA \| Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	100.00%
Embedded derivative - IUL \| Minimum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	37.00%
Embedded derivative - IUL \| Maximum
Assets and liabilities measured at fair value on a recurring basis
Mortality (as a percent)	577.00%
Embedded derivative - Modified coinsurance agreements
Assets and liabilities measured at fair value on a recurring basis
Statutory policy liabilities (net of policy loans)	$ 2,300,000,000
Embedded derivative - Modified coinsurance agreements \| Trading securities
Assets and liabilities measured at fair value on a recurring basis
Fair value	113,000,000
Corporate securities \| Level 3
Assets and liabilities measured at fair value on a recurring basis
Fair value	648,608,000		631,068,000
Financial instruments that are valued using broker quotes	$ 5,200,000		$ 13,500,000