Share-Based Compensation - Stock Options (Details) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) $ / shares shares
Options outstanding and exercisable under the Company's 1991 Equity and Performance Incentive Plan and 2017 Equity and Performance Incentive Plan
Outstanding at January 1, 2019 \| shares	2.5
Outstanding at January 1, 2019 \| $ / shares	$ 27.05
Expired or forfeited \| shares	0.2
Expired or forfeited \| $ / shares	$ 27.84
Granted \| shares	1.2
Granted \| $ / shares	$ 4.67
Outstanding at June 30, 2019 \| shares	3.5
Outstanding at June 30, 2019 \| $ / shares	$ 19.60
Outstanding, weighted-average remaining contractual term	6 years
Outstanding, aggregate intrinsic value \| $	$ 11.0
Options exercisable at June 30, 2019 \| shares	1.8
Options exercisable at June 30, 2019 \| $ / shares	$ 28.81
Options exercisable, weighted-average remaining contractual term	9 years
Options exercisable, aggregate intrinsic value \| $	$ 15.4
Options vested and expected to vest(2) at June 30, 2019 \| shares	1.7
Options vested and expected to vest(2) at June 30, 2019 \| $ / shares	$ 9.80
Options vested and expected to vest, weighted-average remaining contractual term	8 years
Options vested and expected to vest, aggregate intrinsic value \| $	$ 26.4

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Net number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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