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, 2018	Dec. 31, 2017
Investment [Line Items]
Fair Value	$ 2,674,129	$ 2,776,327
Fixed maturities
Investment [Line Items]
Amortized Cost	2,699,826	2,718,904
Gross Unrecognized Holding Gains	30,310	76,586
Gross Unrecognized Holding Losses	(56,007)	(19,163)
Fair Value	2,674,129	2,776,327
Total OTTI Recognized in OCI	0	0
Fixed maturities \| Red Mountain LLC
Investment [Line Items]
Amortized Cost	718,826	704,904
Gross Unrecognized Holding Gains	0	0
Gross Unrecognized Holding Losses	(56,007)	(19,163)
Fair Value	662,819	685,741
Total OTTI Recognized in OCI	0	0
Fixed maturities \| Steel City LLC
Investment [Line Items]
Amortized Cost	1,981,000	2,014,000
Gross Unrecognized Holding Gains	30,310	76,586
Gross Unrecognized Holding Losses	0	0
Fair Value	2,011,310	2,090,586
Total OTTI Recognized in OCI	$ 0	$ 0

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- Definition

Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

+ References

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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- Definition

Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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- Definition

The amount by which the fair value of an investment in debt securities categorized as Held-to-maturity is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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