The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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The price per share of the conversion feature embedded in the debt instrument.

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Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

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Remaining amortization period for discount on the liability component of convertible debt which may be settled in cash upon conversion, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Face (par) amount of debt instrument at time of issuance.

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Description of the frequency of periodic payments (monthly, quarterly, annual).

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

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Date the debt instrument was issued, in CCYY-MM-DD format.

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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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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Fair value amount of debt instrument that was repurchased.

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Amount, after accumulated amortization, of debt discount.

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

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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.

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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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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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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 cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

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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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Long Term Debt Long Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Aug. 01, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt		$ (7,962)	$ 0
Long-term debt	$ 999,198	999,198			$ 601,746
Unamortized Debt Issuance Expense	(7,984)	(7,984)			(7,757)
Debt, Long-term and Short-term, Combined Amount	999,198	999,198			601,746
Current maturities of long-term debt	0	0			0
Total long-term debt, less current portion	999,198	999,198			601,746
Long-term Debt, Fair Value	1,200,000	$ 1,200,000			792,200
6% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		Jun. 27, 2017
Debt Instrument, Face Amount	$ 650,000	$ 650,000
Debt Instrument, Interest Rate, Stated Percentage	6.00%	6.00%
Debt Instrument, Maturity Date		Jul. 15, 2025
Proceeds from Debt, Net of Issuance Costs		$ 636,200
Debt Instrument, Frequency of Periodic Payment		semi-annually
Long-term debt	$ 638,958	$ 638,958			0
Long-term Debt, Fair Value	680,100	680,100
8% Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 250,000
Debt Instrument, Repurchase Amount				$ 265,000
Gain (Loss) on Extinguishment of Debt		8,000
Long-term debt	0	$ 0			247,359
Convertible Debt Securities [Member]
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		Jun. 10, 2014
Debt Instrument, Face Amount	$ 402,500	$ 402,500
Debt Instrument, Interest Rate, Stated Percentage	3.25%	3.25%
Debt Instrument, Maturity Date		Jun. 15, 2029
Debt Instrument, Unamortized Discount	$ 59,000	$ 59,000
Debt Instrument, Convertible, Remaining Discount Amortization Period		3 years 8 months 16 days
Debt Instrument, Frequency of Periodic Payment		semiannually
Common Stock, Conversion Features	14.5645
Debt Instrument, Convertible, Conversion Price \| $ / shares	$ 68.66	$ 68.66
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 37,700	$ 37,700
Long-term debt	368,224	368,224			362,144
Long-term Debt, Fair Value	$ 498,600	$ 498,600			$ 516,800