DEBT AND OTHER OBLIGATIONS - Summary of Debt and Subordinated Debt Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Principal	$ 948,107,000	$ 784,610,000
Carrying Amounts, Debt	1,096,368,000	945,052,000
Carrying Amounts, Subordinated debt securities	495,324,000	495,289,000
Revolving Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	325,000,000	0
Carrying Amounts, Debt	325,000,000	0
Capital lease obligation
Debt Instrument [Line Items]
Outstanding Principal	1,591,000	1,682,000
Carrying Amounts, Debt	1,591,000	1,682,000
Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal	500,000,000	500,000,000
Carrying Amounts, Subordinated debt securities	495,324,000	495,289,000
6.40% Senior Notes (2007), due 2018
Debt Instrument [Line Items]
Outstanding Principal	0	150,000,000
Carrying Amounts, Debt	$ 0	150,518,000
Stated interest rate (as a percent)	6.40%
7.375% Senior Notes (2009), due 2019
Debt Instrument [Line Items]
Outstanding Principal	$ 400,000,000	400,000,000
Carrying Amounts, Debt	$ 431,015,000	435,806,000
Stated interest rate (as a percent)	7.375%
8.45% Senior Notes (2009), due 2039
Debt Instrument [Line Items]
Outstanding Principal	$ 221,516,000	232,928,000
Carrying Amounts, Debt	338,762,000	357,046,000
Debt repurchased and subsequently extinguished	17,500,000
Par value of extinguished debt	$ 11,400,000
Stated interest rate (as a percent)	8.45%
Pre-tax gain (loss) on extinguishment of debt	$ 500,000
5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.35%
5.35% Subordinated Debentures (2017), due 2052 \| Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal	$ 500,000,000	500,000,000
Carrying Amounts, Subordinated debt securities	495,324,000	495,289,000
Deferred issuance expenses		$ 4,800,000
6.00% suordinated debentures due 2042
Debt Instrument [Line Items]
Stated interest rate (as a percent)		6.00%
6.00% suordinated debentures due 2042 \| Subordinated debt securities
Debt Instrument [Line Items]
Debt repurchased and subsequently extinguished		$ 150,000,000
6.25% subordinated debentures due 2042
Debt Instrument [Line Items]
Stated interest rate (as a percent)		6.25%
6.25% subordinated debentures due 2042 \| Subordinated debt securities
Debt Instrument [Line Items]
Debt repurchased and subsequently extinguished		$ 287,500,000
Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	1,000,000,000.0
Line of credit, maximum borrowing capacity to be granted upon entity's request	$ 1,250,000,000.00
Facility fee percentage	0.125%
Line of credit, amount outstanding	$ 325,000,000
Credit Facility \| Federal Funds Rate
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	0.50%
Credit Facility \| One-Month LIBOR
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	1.00%
Credit Facility \| LIBOR
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	1.00%

Debt Instrument, Outstanding Principal

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- Definition

Extinguishment of Debt Par Value

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No definition available.

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- Definition

Maximum borrowing capacity under the credit facility which can be granted upon request by the entity.

+ References

No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

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

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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.

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- Definition

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

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No definition available.

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- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

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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.

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