INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Investments classified as available-for-sale
Amortized Cost	$ 39,783,880	$ 38,233,986
Gross Unrealized Gains	718,229	199,654
Gross Unrealized Losses	(682,181)	(1,862,975)
Fair Value	39,819,928	36,570,665
Total OTTI Recognized in OCI	(28)	(11,039)
Fixed maturities
Investments classified as available-for-sale
Amortized Cost	38,489,362	37,192,864
Gross Unrealized Gains	695,911	191,903
Gross Unrealized Losses	(673,410)	(1,841,290)
Fair Value	38,511,863	35,543,477
Total OTTI Recognized in OCI	(28)	(11,039)
Residential mortgage-backed securities
Investments classified as available-for-sale
Amortized Cost	2,330,832	1,913,413
Gross Unrealized Gains	19,413	10,737
Gross Unrealized Losses	(23,033)	(25,667)
Fair Value	2,327,212	1,898,483
Total OTTI Recognized in OCI	10	(9)
Commercial mortgage-backed securities
Investments classified as available-for-sale
Amortized Cost	1,914,998	1,850,620
Gross Unrealized Gains	5,010	2,528
Gross Unrealized Losses	(30,186)	(41,678)
Fair Value	1,889,822	1,811,470
Other asset-backed securities
Investments classified as available-for-sale
Amortized Cost	1,234,376	1,210,490
Gross Unrealized Gains	20,936	21,741
Gross Unrealized Losses	(5,763)	(20,698)
Fair Value	1,249,549	1,211,533
U.S. government-related securities
Investments classified as available-for-sale
Amortized Cost	1,255,244	1,308,192
Gross Unrealized Gains	185	422
Gross Unrealized Losses	(32,177)	(40,455)
Fair Value	1,223,252	1,268,159
Other government-related securities
Investments classified as available-for-sale
Amortized Cost	282,767	253,182
Gross Unrealized Gains	9,463	1,536
Gross Unrealized Losses	(4,948)	(14,797)
Fair Value	287,282	239,921
States, municipals, and political subdivisions
Investments classified as available-for-sale
Amortized Cost	1,770,299	1,760,837
Gross Unrealized Gains	16,959	1,224
Gross Unrealized Losses	(45,613)	(105,558)
Fair Value	1,741,645	1,656,503
Total OTTI Recognized in OCI	(37)
Corporate securities
Investments classified as available-for-sale
Amortized Cost	29,606,484	28,801,768
Gross Unrealized Gains	623,713	153,715
Gross Unrealized Losses	(528,187)	(1,583,918)
Fair Value	29,702,010	27,371,565
Total OTTI Recognized in OCI	(1)	(11,030)
Redeemable preferred stock
Investments classified as available-for-sale
Amortized Cost	94,362	94,362
Gross Unrealized Gains	232	0
Gross Unrealized Losses	(3,503)	(8,519)
Fair Value	91,091	85,843
Equity securities
Investments classified as available-for-sale
Amortized Cost	735,569	761,340
Gross Unrealized Gains	22,318	7,751
Gross Unrealized Losses	(8,771)	(21,685)
Fair Value	749,116	747,406
Short-term investments
Investments classified as available-for-sale
Amortized Cost	558,949	279,782
Fair Value	$ 558,949	$ 279,782

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25362-111560

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

+ Details

Name:	us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterestAvailableforsaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_PreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: