Long-Term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,563,931	$ 3,121,558
Unamortized discount	(1,428)	(1,556)
Unamortized Debt Issuance Expense	(50,909)	(44,122)
Long-term Debt, Current Maturities	23,981	23,981
Long-term debt, gross, excluding current maturities	3,539,950	3,097,577
Long-term debt, net of current maturities and debt issuance costs	3,487,613	3,051,899
Parent
Debt Instrument [Line Items]
Long-term Debt, Gross	3,563,931	3,121,558
Unamortized discount	(1,428)	(1,556)
Unamortized Debt Issuance Expense	(50,909)	(44,122)
Long-term debt, net	3,511,594	3,075,880
Long-term Debt, Current Maturities	23,895	23,895
Long-term debt, net of current maturities and debt issuance costs	$ 3,487,244	$ 3,051,481
Parent \| Line of Credit \| Bank Credit Facility [Member]
Debt Instrument [Line Items]
Interest rate at period end	4.45%	3.882%
Unamortized discount	$ (1,428)	$ (1,556)
Unamortized Debt Issuance Expense	(21,091)	(23,795)
Parent \| Senior Notes \| 6.875 % Senior Notes Due 2023
Debt Instrument [Line Items]
Unamortized discount	0	0
Unamortized Debt Issuance Expense	$ (8,578)	$ (9,455)
Debt Instrument, Interest Rate, Stated Percentage	6.875%	6.875%
Parent \| Senior Notes \| 6.375% Senior Notes Due 2026
Debt Instrument [Line Items]
Unamortized discount	$ 0	$ 0
Unamortized Debt Issuance Expense	$ (10,213)	$ (10,872)
Debt Instrument, Interest Rate, Stated Percentage	6.375%	6.375%
Parent \| Senior Notes \| 6.000% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Unamortized discount	$ 0
Unamortized Debt Issuance Expense	$ (11,027)
Debt Instrument, Interest Rate, Stated Percentage	6.00%
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	$ 1,363,477	$ 1,621,054
Long-term debt, net	1,340,958	1,595,703
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	741,422	740,545
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	739,787	739,128
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Senior Notes \| 6.000% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	700,000
Long-term debt, net	688,973
Fair Value, Measurements, Nonrecurring [Member] \| Parent \| Other Debt Obligations [Member] \| Other
Debt Instrument [Line Items]
Long-term Debt, Gross	454	504
Long-term debt, net	$ 454	$ 504

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
-Paragraph 1
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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
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

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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
-Topic 944
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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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