DEBT AND OTHER OBLIGATIONS - Summary of Debt and Subordinated Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding Principal	$ 784,610	$ 966,926
Total debt	945,052	1,163,285
Subordinated debt securities	495,289	441,202
6.40% Senior Notes (2007), due 2018
Debt Instrument [Line Items]
Outstanding Principal	150,000	150,000
Total debt	$ 150,518	156,663
Stated interest rate (as a percent)	6.40%
7.375% Senior Notes (2009), due 2019
Debt Instrument [Line Items]
Outstanding Principal	$ 400,000	400,000
Total debt	$ 435,806	454,688
Stated interest rate (as a percent)	7.375%
8.45% Senior Notes (2009), due 2039
Debt Instrument [Line Items]
Outstanding Principal	$ 232,928	246,926
Total debt	$ 357,046	$ 381,934
Stated interest rate (as a percent)	8.45%	8.45%
6.25% Subordinated Debentures (2012), due 2042, callable 2017
Debt Instrument [Line Items]
Subordinated debt securities	$ 0	$ 290,002
Stated interest rate (as a percent)	6.25%
6.00% Subordinated Debentures (2012), due 2042, callable 2017
Debt Instrument [Line Items]
Subordinated debt securities	$ 0	151,200
Stated interest rate (as a percent)	6.00%
5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Subordinated debt securities	$ 495,289	0
Stated interest rate (as a percent)	5.35%
Credit Facility
Debt Instrument [Line Items]
Outstanding Principal	$ 0	170,000
Total debt	0	170,000
Capital lease obligation
Debt Instrument [Line Items]
Outstanding Principal	1,682	0
Total debt	1,682	0
Subordinated debentures
Debt Instrument [Line Items]
Outstanding Principal	500,000	437,500
Subordinated debentures \| 6.25% Subordinated Debentures (2012), due 2042, callable 2017
Debt Instrument [Line Items]
Outstanding Principal	0	287,500
Subordinated debentures \| 6.00% Subordinated Debentures (2012), due 2042, callable 2017
Debt Instrument [Line Items]
Outstanding Principal	0	150,000
Subordinated debentures \| 5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Outstanding Principal	$ 500,000	$ 0

Debt Instrument, Outstanding Principal

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ 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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: 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(19))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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