Stock Compensation Plans - Total Share Awards (Details) - $ / shares shares in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average per share fair value of discount (in USD per share)	$ 2.56	$ 2.63	$ 2.26
Restricted Stock and Restricted Stock Units
Number of Shares (in thousands)
Non-vested at beginning of year (in shares)	4,444
Granted (in shares)	1,359
Vested (in shares)	(1,721)
Forfeited (in shares)	(636)
Non-vested at end of year (in shares)	3,446	4,444
Weighted-Average Grant-Date Fair Value
Non-vested at beginning of year (in USD per share)	$ 43.94
Granted (in USD per share)	53.11	$ 48.90	42.87
Vested (in USD per share)	41.52
Forfeited (in USD per share)	46.07
Non-vested at end of year (in USD per share)	$ 48.43	$ 43.94
Performance Shares
Number of Shares (in thousands)
Non-vested at beginning of year (in shares)	795
Granted (in shares)	372
Performance based adjustment (in shares)	188
Vested (in shares)	(539)
Forfeited (in shares)	(81)
Non-vested at end of year (in shares)	735	795
Weighted-Average Grant-Date Fair Value
Non-vested at beginning of year (in USD per share)	$ 45.16
Granted (in USD per share)	50.26	$ 48.89	$ 41.50
Performance based adjustment (in USD per share)	43.59
Vested (in USD per share)	43.59
Forfeited (in USD per share)	44.45
Non-vested at end of year (in USD per share)	$ 49.56	$ 45.16

The weighted average per share fair value of the discount under the ESPP plan.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

+ References

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The net total number of shares (or other type of equity) under an equity-based award plan, other than a stock option plan, that were granted, vested and forfeited during the reporting period.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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Change in the weighted average exercise price of options outstanding.

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