Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Face (par) amount of debt instrument at time of issuance.

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

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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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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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LONG-TERM DEBT - Other Information (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Nov. 16, 2017
Unsecured debt
Unsecured notes	$ 37,900
Credit facility information
Outstanding borrowings	3,201,538	$ 3,211,421
First Mortgage Obligations, Series 2017A
Bonds
Face amount of debt			$ 120,000
First Mortgage Obligations, Series 2017A , Tranche 1, 3.34%, due through 2029
Bonds
Face amount of debt			$ 60,000
Credit facility information
Stated interest rate (as a percent)			3.34%
First Mortgage Obligations, Series 2017A, Tranche 2, 3.39%, due December 12, 2029
Bonds
Face amount of debt			$ 60,000
Credit facility information
Stated interest rate (as a percent)			3.39%
Revolving Credit Agreement
Credit facility information
Maximum borrowing capacity	650,000
Outstanding borrowings	0
Available borrowing capacity	428,000
Commercial Paper \| Revolving Credit Agreement
Credit facility information
Available borrowing capacity	$ 278,000