INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Held-to-Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Fair Value	$ 2,796,603	$ 2,733,340
Fixed maturities
Investment [Line Items]
Amortized Cost	2,730,995	2,770,177
Gross Unrecognized Holding Gains	82,913	30,385
Gross Unrecognized Holding Losses	(17,305)	(67,222)
Fair Value	2,796,603	2,733,340
Total OTTI Recognized in OCI	0	0
Red Mountain LLC \| Fixed maturities
Investment [Line Items]
Amortized Cost	690,995	654,177
Gross Unrecognized Holding Gains	0	0
Gross Unrecognized Holding Losses	(17,305)	(67,222)
Fair Value	673,690	586,955
Total OTTI Recognized in OCI	0	0
Steel City LLC \| Fixed maturities
Investment [Line Items]
Amortized Cost	2,040,000	2,116,000
Gross Unrecognized Holding Gains	82,913	30,385
Gross Unrecognized Holding Losses	0	0
Fair Value	2,122,913	2,146,385
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.

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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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