Stock-Based Compensation - Changes in Stock Options Outstanding (Details) - $ / shares shares in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Share Option Activity
Outstanding, beginning balance (in shares)	7,841
Granted (in shares)	1,248
Exercised (in shares)	(683)
Forfeited (in shares)	(184)
Expired (in shares)	(684)
Outstanding, ending balance (in shares)	7,538	7,841
Stock Options Weighted Average Exercise Price
Outstanding, beginning (in dollars per share)	$ 35.59
Granted (in dollars per share)		$ 35.53
Exercised (in dollars per share)		25.59
Forfeited (in dollars per share)		36.59
Expired (in dollars per share)		54.41
Outstanding, ending (in dollars per share)	$ 34.76	$ 35.59
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options exercisable (in shares)	5,389
Options exercisable (in dollars per share)	$ 34.27

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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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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

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