Stock-based Compensation - Award Activity (Details) - shares shares in Thousands	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Equity awards outstanding, beginning of year (in shares)	4,757
Granted (in shares)	1,675
Exercised (in shares)	(1,140)
Expired or forfeited (in shares)	(949)
Equity awards outstanding, end of period (in shares)	4,343
RSUs
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Equity awards outstanding, beginning of year (in shares)	58	15
Granted (in shares)	58
Exercised (in shares)	(15)
Expired or forfeited (in shares)	0
Equity awards outstanding, end of period (in shares)	58
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Equity awards outstanding, beginning of year (in shares)	746	538
Granted (in shares)	650
Exercised (in shares)	0
Expired or forfeited (in shares)	(442)
Equity awards outstanding, end of period (in shares)	746

Number of non-option equity instruments exercised by participants.

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Number of shares under non-option equity instrument agreements that were either cancelled or expired.

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Net number of non-option equity instruments granted to participants.

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Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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For presentations that combine terminations, 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 or that expired.

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

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

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Number of share options (or share units) exercised during the current period.

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