Long-Term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,118,131	$ 3,283,129
Unamortized discount	1,613	1,888
Unamortized Debt Issuance Expense	46,951	51,786
Current maturities of long-term debt	23,981	30,336
Long-term debt, gross, excluding current maturities	3,094,150	3,252,793
Long-term debt, net of current maturities and debt issuance costs	3,045,586	3,199,119
Parent
Debt Instrument [Line Items]
Long-term Debt, Gross	3,118,131	3,283,129
Unamortized discount	1,613	1,888
Unamortized Debt Issuance Expense	46,951	51,786
Long-term debt, net	3,069,567	3,229,455
Current maturities of long-term debt	23,894	30,250
Long-term debt, net of current maturities and debt issuance costs	$ 3,045,168	$ 3,198,613
Parent \| Line of Credit \| Bank Credit Facility [Member]
Debt Instrument [Line Items]
Interest rate at period end	3.516%	3.44%
Unamortized discount	$ 1,613	$ 1,888
Unamortized Debt Issuance Expense	25,856	28,503
Parent \| Senior Notes \| 6.875 % Senior Notes Due 2023
Debt Instrument [Line Items]
Unamortized discount	0	0
Unamortized Debt Issuance Expense	$ 11,201	$ 11,209
Debt Instrument, Interest Rate, Stated Percentage	6.875%	6.88%
Parent \| Senior Notes \| 6.375% Senior Notes Due 2026
Debt Instrument [Line Items]
Unamortized discount	$ 0	$ 0
Unamortized Debt Issuance Expense	$ 9,894	$ 12,074
Debt Instrument, Interest Rate, Stated Percentage	6.375%	6.38%
Parent \| Other Debt Obligations [Member] \| Other
Debt Instrument [Line Items]
Unamortized discount	$ 0	$ 0
Unamortized Debt Issuance Expense	$ 0	$ 0
Debt Instrument, Interest Rate, Stated Percentage	5.80%	5.80%
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Line of Credit \| Bank Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,617,628	$ 1,782,538
Long-term debt, net	1,590,159	1,752,147
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Senior Notes \| 6.875 % Senior Notes Due 2023
Debt Instrument [Line Items]
Long-term Debt, Gross	750,000	750,000
Long-term debt, net	738,799	738,791
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Senior Notes \| 6.375% Senior Notes Due 2026
Debt Instrument [Line Items]
Long-term Debt, Gross	750,000	750,000
Long-term debt, net	740,106	737,926
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Other Debt Obligations [Member] \| Other
Debt Instrument [Line Items]
Long-term Debt, Gross	503	591
Long-term debt, net	$ 503	$ 591

Long-term Debt, Gross, Excluding Current Maturities

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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.

+ References

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

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Amount, after accumulated amortization, of debt discount.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
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The effective interest rate at the end of the reporting period.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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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.

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