INVESTMENT OPERATIONS - Schedule of Amortized Cost and Fair Value of the Company's Investments Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Amortized Cost	$ 39,306,705	$ 37,991,304
Gross Unrealized Gains	562,657	197,239
Gross Unrealized Losses	(861,709)	(1,859,707)
Fair Value	39,007,653	36,328,836
Total OTTI Recognized in OCI	(77)	(11,039)
Residential mortgage-backed securities
Investment [Line Items]
Amortized Cost	2,178,344	1,904,165
Gross Unrealized Gains	22,141	10,737
Gross Unrealized Losses	(17,116)	(25,295)
Fair Value	2,183,369	1,889,607
Total OTTI Recognized in OCI	16	(9)
Commercial mortgage-backed securities
Investment [Line Items]
Amortized Cost	1,909,921	1,820,644
Gross Unrealized Gains	6,052	2,455
Gross Unrealized Losses	(23,920)	(40,602)
Fair Value	1,892,053	1,782,497
Total OTTI Recognized in OCI	0	0
Other asset-backed securities
Investment [Line Items]
Amortized Cost	1,186,463	1,210,490
Gross Unrealized Gains	17,692	21,741
Gross Unrealized Losses	(13,488)	(20,698)
Fair Value	1,190,667	1,211,533
Total OTTI Recognized in OCI	0	0
U.S. government-related securities
Investment [Line Items]
Amortized Cost	1,317,910	1,308,192
Gross Unrealized Gains	1,256	422
Gross Unrealized Losses	(26,333)	(40,455)
Fair Value	1,292,833	1,268,159
Total OTTI Recognized in OCI	0	0
Other government-related securities
Investment [Line Items]
Amortized Cost	251,040	251,197
Gross Unrealized Gains	8,075	1,526
Gross Unrealized Losses	(6,835)	(14,797)
Fair Value	252,280	237,926
Total OTTI Recognized in OCI	0	0
States, municipals, and political subdivisions
Investment [Line Items]
Amortized Cost	1,756,865	1,760,837
Gross Unrealized Gains	6,416	1,224
Gross Unrealized Losses	(59,272)	(105,558)
Fair Value	1,704,009	1,656,503
Total OTTI Recognized in OCI	(93)	0
Corporate securities
Investment [Line Items]
Amortized Cost	29,511,875	28,655,364
Gross Unrealized Gains	475,788	151,383
Gross Unrealized Losses	(704,556)	(1,582,098)
Fair Value	29,283,107	27,224,649
Total OTTI Recognized in OCI	0	(11,030)
Preferred stock
Investment [Line Items]
Amortized Cost	94,362	94,362
Gross Unrealized Gains	376	0
Gross Unrealized Losses	(2,898)	(8,519)
Fair Value	91,840	85,843
Total OTTI Recognized in OCI	0	0
Fixed maturities
Investment [Line Items]
Amortized Cost	38,206,780	37,005,251
Gross Unrealized Gains	537,796	189,488
Gross Unrealized Losses	(854,418)	(1,838,022)
Fair Value	37,890,158	35,356,717
Total OTTI Recognized in OCI	(77)	(11,039)
Equity securities
Investment [Line Items]
Amortized Cost	721,496	722,868
Gross Unrealized Gains	24,861	7,751
Gross Unrealized Losses	(7,291)	(21,685)
Fair Value	739,066	708,934
Total OTTI Recognized in OCI		0
Short-term investments
Investment [Line Items]
Amortized Cost	378,429	263,185
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	378,429	263,185
Total OTTI Recognized in OCI		$ 0

No definition available.

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

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

- 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_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_FixedMaturitiesMember
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: