Debt (Schedule of Obligations Under Debt Arrangements) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 11, 2017
Debt Instrument [Line Items]
Senior secured credit facility, principal	$ 1,306,300	$ 1,099,200
Unamortized Debt Issuance Expense and Discounts (Premiums)	41,386	46,252
Senior unsecured notes	2,458,614	2,598,918
Total long-term debt, principal	3,806,300	3,744,370
Total long-term debt	$ 3,764,914	$ 3,698,118
5.750% senior unsecured notes due February 2021
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	5.75%	5.75%	5.75%
Principal	$ 0	$ 145,170
Unamortized Debt Issuance Expense and Discounts (Premiums)	0	1,303
Senior unsecured notes	$ 0	$ 143,867
6.750% senior unsecured notes due August 2022
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	6.75%	6.75%
Principal	$ 750,000	$ 750,000
Unamortized Debt Issuance Expense and Discounts (Premiums)	14,433	16,077
Senior unsecured notes	$ 735,567	$ 733,923
6.000% senior unsecured notes due May 2023
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	6.00%	6.00%
Principal	$ 400,000	$ 400,000
Unamortized Debt Issuance Expense and Discounts (Premiums)	5,158	5,691
Senior unsecured notes	$ 394,842	$ 394,309
5.625% senior unsecured notes due June 2024
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	5.625%	5.625%
Principal	$ 350,000	$ 350,000
Unamortized Debt Issuance Expense and Discounts (Premiums)	5,268	5,717
Senior unsecured notes	$ 344,732	$ 344,283
6.500% senior unsecured notes due October 2025
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	6.50%	6.50%
Principal	$ 550,000	$ 550,000
Unamortized Debt Issuance Expense and Discounts (Premiums)	8,851	9,462
Senior unsecured notes	$ 541,149	$ 540,538
6.250% senior unsecured notes due May 2026
Debt Instrument [Line Items]
Senior unsecured notes, stated rate	6.25%	6.25%	6.25%
Principal	$ 450,000	$ 450,000	$ 450,000
Unamortized Debt Issuance Expense and Discounts (Premiums)	7,676	8,002
Senior unsecured notes	442,324	441,998
Senior Secured Credit Facility
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)	12,400	14,100
Line of Credit
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)	$ 0	$ 0

Long-term debt, Face Values before unamortized discount and debt issuance costs

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset. Also includes the amount of debt discount (premium) that was originally recognized at the issuance of the instrument that has yet to be amortized.

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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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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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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Carrying amount of collateralized debt obligations with maturities initially due after one year or beyond the operating cycle, if longer, excluding the current portion. Obligations include, but not limited to, mortgage loans, chattel loans, and other borrowings secured by assets.

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Carrying value as of the balance sheet date of uncollateralized debt obligation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion.

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