Convertible Senior Notes - Additional Information (Details)	1 Months Ended	12 Months Ended
Nov. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) d $ / shares
Debt Instrument [Line Items]
Difference between aggregate principal amount and estimated fair value of liability component recoded in additional paid-in capital		$ 22,094,000
Convertible Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount	$ 115,000,000	$ 115,000,000
Debt instrument, interest rate	1.50%
Debt instrument maturity date	Nov. 01, 2022
Debt instrument, interest rate terms		Interest is payable semiannually in arrears on May 1 and November 1 of each year, commencing on May 1, 2018.
Interest payment commencing date	May 01, 2018
Debt Conversion, Converted Instrument, Shares Issued \| shares	29.6626
Principal amount per note used in conversion rate	$ 1,000
Conversion price per share \| $ / shares	$ 33.71
Proceeds from issuance of convertible note	$ 3,400,000
Descripition of convertible notes at option of holders		Prior to the close of business on the business day immediately preceding May 1, 2022, the Notes will be convertible at the option of holders during certain periods, only upon satisfaction of certain conditions set forth below. On or after May 1, 2022, until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert all or any portion of their Notes at the conversion rate at any time regardless of whether the conditions set forth below have been met
Implied interest rate		6.93%
Debt instrument, fair value		$ 92,100,000
Difference between aggregate principal amount and estimated fair value of liability component recoded in additional paid-in capital		22,900,000
Transaction costs attributable to liability component		3,200,000
Transaction costs attributable to equity component		$ 800,000
Initial strike price of capped call options \| $ / shares		$ 33.71
Capped call options, cap price \| $ / shares		$ 47.20
Cost of purchased capped call options		$ 12,900,000
Share price \| $ / shares		$ 29.72
Convertible Senior Notes \| Scenario One
Debt Instrument [Line Items]
Number of trading days \| d		20
Number of consecutive trading days \| d		30
Percentage of last reported sale price to conversion price on each applicable trading day		130.00%
Convertible Senior Notes \| Scenario Two
Debt Instrument [Line Items]
Number of trading days \| d		5
Percentage of last reported sale price to conversion price on each applicable trading day		98.00%
Notes measurement period		5 days

Capped call options cap price.

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Debt issuance costs attributable to equity component.

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Principal amount per note used in conversion rate.

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Adjustment to additional paid in capital resulting from the recognition of convertible debt instruments as two separate components - a debt component and an equity component. This bifurcation may result in a basis difference associated with the liability component that represents a temporary difference for purposes of applying accounting for income taxes. The initial recognition of deferred taxes for the tax effect of that temporary difference is as an adjustment to additional paid in capital.

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The value of the financial instrument issued [noncash or part noncash] in the conversion of stock. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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The number of shares issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period.

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

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Price of the entity's common stock which would be required to be attained for the conversion feature embedded in the debt instrument to become effective.

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Threshold period of specified consecutive trading days within which common stock price to conversion price of convertible debt instrument must exceed threshold percentage for specified number of trading days to trigger conversion feature.

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Minimum percentage of common stock price to conversion price of convertible debt instruments to determine eligibility of conversion.

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Threshold number of specified trading days that common stock price to conversion price of convertible debt instruments must exceed threshold percentage within a specified consecutive trading period to trigger conversion feature.

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Date the debt agreement requires the first payment to be made, in CCYY-MM-DD format.

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

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

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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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Price of a single share of a number of saleable stocks of a company.

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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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