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.

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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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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 accumulated amortization, of debt issuance costs related to line of credit arrangements. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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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Debt - Additional Information (Details)	Aug. 20, 2018 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($)	Jun. 26, 2015 USD ($)
Debt Instrument [Line Items]
Principal amount		$ 336,250,000	$ 316,250,000
Conversion price (in dollars per share) \| $ / shares		$ 101.38
Unamortized debt issuance costs		$ 5,966,000	6,681,000
Line of Credit
Debt Instrument [Line Items]
Borrowing limit used		1,000,000.0
Convertible Note Due 2023
Debt Instrument [Line Items]
Principal amount		$ 316,250,000	316,250,000
Convertible Note Due 2023 \| Senior Notes
Debt Instrument [Line Items]
Principal amount	$ 316,300,000
Interest rate (as a percent)		2.00%
Debt conversion ratio (in shares)	0.0098643
Conversion price (in dollars per share) \| $ / shares	$ 101.38
Credit facility
Debt Instrument [Line Items]
Principal amount		$ 20,000,000	0
Credit facility \| Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity					$ 550,000,000.0
Debt issuance costs				$ 1,700,000
Borrowings outstanding			0
Unamortized debt issuance costs		$ 2,800,000	$ 3,600,000