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Actuarially determined death rate of policyholders, which is included in determining the amount of the specified guaranteed benefit obligation as of the balance sheet date, as a percentage of the 94 GMDB table.

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Represent the payments triggered under the agreement.

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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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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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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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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	12 Months Ended
	Mar. 31, 2018 USD ($) quote primary_source input_method	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Determination of fair values
Number of sources of information considered \| primary_source	1
Number of sources of information \| input_method	3
Percentage of the Company's fixed maturity securities priced by third party pricing services	92.40%
Number of broker quotes obtained per security \| quote	1
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program		97.00%
Percentage of derivatives excluding embedded derivatives that were priced using exchange prices or independent broker quotations	82.60%
Fair value, assets	$ 618,772,000	$ 629,537,000
Financial instruments owned, other	65,500,000	65,500,000
Interest support, YRT premium support and portfolio maintenance agreement
Determination of fair values
Fair value, assets	103,100,000
Golden Gate II \| Interest Support Agreement
Determination of fair values
Fair value, assets	48,800,000
Payments made under agreement	0
Amount of collateralized support agreement obligations by PLC	4,800,000
Golden Gate II \| YRT premium support agreement
Determination of fair values
Fair value, assets	49,400,000
Payments made under agreement	0
Golden Gate V and West Coast Life \| Portfolio maintenance agreements
Determination of fair values
Fair value, assets	4,800,000
Payments made under agreement	0
Level 3
Determination of fair values
Level 3 fair value	569,995,000	681,034,000	$ 525,216,000	$ 493,419,000
Financial instruments valued in broker quotes	49,100,000	50,000,000
Level 3 \| Annuity account
Determination of fair values
Level 3 fair value	81,400,000
Asset-backed securities \| Level 2
Determination of fair values
Fair value	5,400,000,000
Asset-backed securities \| Level 3
Determination of fair values
Fair value	$ 538,700,000
Other asset-backed securities
Determination of fair values
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Other asset-backed securities \| Level 3
Determination of fair values
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Financial instruments valued in broker quotes	$ 35,100,000	35,400,000
Other asset-backed securities \| Level 3 \| Available-for-sale securities
Determination of fair values
Fair value	503,800,000
Other asset-backed securities \| Level 3 \| Trading securities
Determination of fair values
Fair value	34,900,000
Corporate securities \| Level 3
Determination of fair values
Financial instruments valued in broker quotes	14,000,000	$ 14,600,000
Corporate securities \| Level 2
Determination of fair values
Fair value	32,100,000,000
Corporate securities \| Level 3
Determination of fair values
Fair value	631,700,000
Equity securities
Determination of fair values
Fair value	65,500,000
Federal Home Loan Bank stock	$ 65,500,000
Embedded derivative - GLWB \| Level 3 \| Minimum
Determination of fair values
Mortality (as a percent)	91.10%
Embedded derivative - GLWB \| Level 3 \| Maximum
Determination of fair values
Mortality (as a percent)	106.60%
Embedded derivative - FIA \| Level 3 \| Minimum
Determination of fair values
Mortality (as a percent)	46.00%
Embedded derivative - FIA \| Level 3 \| Maximum
Determination of fair values
Mortality (as a percent)	113.00%
Embedded derivative - IUL \| Level 3 \| Minimum
Determination of fair values
Mortality (as a percent)	34.00%
Embedded derivative - IUL \| Level 3 \| Maximum
Determination of fair values
Mortality (as a percent)	152.00%
Embedded derivative - Modified coinsurance agreements
Determination of fair values
Statutory policy liabilities (net of policy loans)	$ 2,400,000,000
Embedded derivative - Modified coinsurance agreements \| Trading securities
Determination of fair values
Fair value	134,100,000
Funds withheld derivative \| Level 2
Determination of fair values
Fair value, liabilities	$ 101,800,000