Convertible Senior Notes - Convertible Senior Notes Due 2023 (Narrative) (Details) - Convertible Senior Notes Due 2023 - Senior Notes	1 Months Ended	9 Months Ended
Jun. 30, 2016 USD ($) day $ / shares	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Principal	$ 402,500,000	$ 402,500,000	$ 402,500,000
Interest rate	2.75%
Proceeds from offering	$ 390,800,000
Trading days \| day	20
Redemption consecutive trading period	30 days
Redemption stock price conversion threshold (greater than or equal to)	130.00%
Consecutive measurement period	5 days
Percent of trading price (less than)	98.00%
Consecutive trading days \| day	50
Conversion ratio	0.0204198
Conversion price (USD per share) \| $ / shares	$ 48.97
Redemption trading period	19 days
Percent of principal amount plus accrued and unpaid interest	100.00%
Debt default principal amount percentage	25.00%
Debt instrument, term	7 years
Carrying amount, equity component	$ 101,000,000
Net deferred tax liabilities	$ 33,500,000
Remaining contractual life		4 years 9 months
Significant Other Observable Inputs (Level 2)
Debt Instrument [Line Items]
Fair value of notes		$ 371,600,000

Debt Instrument, Convertible, Consecutive Measurement Period

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- Definition

Debt Instrument, Covenant, Percent of Principal Amount, Plus Accrued and Unpaid Interest

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Debt Instrument, Covenant, Percent of Trading Price

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Debt Instrument, Debt Default, Principal Amount Percentage

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Debt Instrument, Redemption, Threshold Consecutive Trading Days

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Debt Instrument, Redemption, Threshold Percentage of Stock Price Trigger

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Debt Instrument, Redemption, Threshold Trading Days

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- Definition

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

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- Definition

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

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- Definition

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

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- Definition

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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- Definition

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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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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- Definition

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

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- Definition

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

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- Definition

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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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences without jurisdictional netting.

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- Definition

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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The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

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