Long Term Debt Long Term Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Fair Value	$ 1,217,000	$ 1,102,000
Debt Instrument, Unamortized Discount	30,647	33,191
Unamortized Debt Issuance Expense	(5,886)	(6,180)
Debt, Long-term and Short-term, Combined Amount	1,015,967	1,013,129
Current maturities of long-term debt	0	0
Total long-term debt, less current portion	$ 1,015,967	1,013,129
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%
Debt Instrument, Frequency of Periodic Payment	semi-annually
Debt Instrument, Maturity Date	Jul. 15, 2025
Long-term Debt, Fair Value	$ 677,600	645,500
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%
Debt Instrument, Frequency of Periodic Payment	semiannually
Debt Instrument, Maturity Date	Jun. 15, 2029
Common Stock, Conversion Features	14.7316
Debt Instrument, Convertible, Conversion Price \| $ / shares	$ 67.88
Debt Instrument, Convertible, Carrying Amount of Equity Component	$ 37,700
Debt Instrument, Convertible, Remaining Discount Amortization Period	2 years 2 months 16 days
Long-term Debt, Fair Value	$ 539,100	$ 457,000
Debt Instrument, Unamortized Discount	$ 59,000

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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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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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 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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