INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Held-to-Maturity (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Fair Value	$ 2,594,441	$ 2,547,210
Fixed maturities
Investment [Line Items]
Amortized Cost	2,607,356	2,633,474
Gross Unrecognized Holding Gains	43,568	0
Gross Unrecognized Holding Losses	(56,483)	(86,264)
Fair Value	2,594,441	2,547,210
Total OTTI Recognized in OCI	0	0
Fixed maturities \| Red Mountain, LLC
Investment [Line Items]
Amortized Cost	764,356	750,474
Gross Unrecognized Holding Gains	0	0
Gross Unrecognized Holding Losses	(56,483)	(81,657)
Fair Value	707,873	668,817
Total OTTI Recognized in OCI	0	0
Fixed maturities \| Steel City, LLC
Investment [Line Items]
Amortized Cost	1,843,000	1,883,000
Gross Unrecognized Holding Gains	43,568	0
Gross Unrecognized Holding Losses	0	(4,607)
Fair Value	1,886,568	1,878,393
Total OTTI Recognized in OCI	$ 0	$ 0

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- Definition

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

+ References

No definition available.

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at amortized cost (held-to-maturity), recognized in earnings and other comprehensive income (OCI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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