Maximum Percentage of Shares Purchased from Annual Compensation.

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Number of peer groups.

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Number of Performance-Based Programs

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Percentage Of Target Amount Granted Maximum

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Percentage Of Target Amount Granted Minimum

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Performance Based Long Term Incentive Program Weighted Percentage

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Rate of Discount given to Employees.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Shares Earned Maximum

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Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Shares Earned Minimum

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Grants in Period

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Share-based Compensation Arrangement by Share-based Payment Award, Performance Period

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Aggregate number of common shares reserved for future issuance.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

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Discount rate from fair value on purchase date that participants pay for shares.

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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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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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

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Price of a single share of a number of saleable stocks of a company.

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Percentage of vesting of award under share-based payment arrangement.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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Share-Based Compensation (Narrative) (Details)	3 Months Ended			9 Months Ended		12 Months Ended
Share-Based Compensation (Narrative) (Details)	Sep. 30, 2020 USD ($) $ / shares peer_group shares	Mar. 31, 2020 shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) program $ / shares peer_group shares	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance (in shares) \| shares	9,700,000			9,700,000
Net cash proceeds from the exercise of stock options \| $	$ 1,000,000			$ 2,000,000	$ 1,000,000
Stock option exercises, net (in shares) \| shares	25,113		9,216	80,378	62,624
Stock options, exercisable (in shares) \| shares	298,289		300,000	298,289	300,000
Options exercisable, weighted average exercise price (in dollars per share) \| $ / shares	$ 62.59		$ 49.76	$ 62.59	$ 49.76
Total pre-tax intrinsic value of stock options exercised \| $	$ 2,000,000		$ 1,000,000	$ 8,000,000	$ 5,000,000
Compensation expenses \| $	$ 24,000,000		21,000,000	$ 64,000,000	$ 57,000,000
Commercial Paper and Letter Of Credit \| Clearinghouse Credit Facilities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share price (in dollars per share) \| $ / shares	$ 122.71			$ 122.71
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount from market price (as a percent)				15.00%	15.00%
Common stock shares reserved for future issuance (in shares) \| shares	4,500,000			4,500,000
Maximum percentage of shares purchased from annual compensation				10.00%
Discount given to employees (as a percent)				15.00%
Compensation expenses \| $	$ 1,000,000		$ 1,000,000	$ 4,000,000	$ 3,000,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost \| $	79,000,000			$ 79,000,000
Weighted-average period unrecognized compensation cost is expected to be recognized, in years				1 year 9 months 18 days
PSUs \| Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of performance-based programs (in programs) \| program				2
PSUs \| One-Year Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost \| $	6,000,000			$ 6,000,000
Weighted-average period unrecognized compensation cost is expected to be recognized, in years				1 year 3 months 18 days
Expiration period of PSU program				1 year		1 year
Percentage of target amount granted, minimum				0.00%
Percentage of target amount granted, maximum				150.00%
Additional units granted above target (in shares) \| shares		26,780
PSUs \| Three-Year Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost \| $	$ 39,000,000			$ 39,000,000
Weighted-average period unrecognized compensation cost is expected to be recognized, in years				1 year 6 months
Expiration period of PSU program				3 years
Performance period				3 years
Additional units granted above target (in shares) \| shares		43,684
Number of peer groups (in peer groups) \| peer_group	2			2
Performance-based long-term incentive program weighted percentage				50.00%
Minimum payout (as a percent)				0.00%
Maximum payout (as a percent)				200.00%
Share price (in dollars per share) \| $ / shares	$ 92.34		$ 89.00	$ 92.34	$ 89.00
PSUs, Negative TSR \| Three-Year Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum payout (as a percent)				100.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares) \| shares					0
Second Anniversary \| Restricted Stock \| Below Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				25.00%
Second Anniversary \| Restricted Stock \| At or Above Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				33.30%
Third Anniversary \| Restricted Stock \| Below Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				25.00%
Third Anniversary \| Restricted Stock \| At or Above Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				33.30%
Fourth Anniversary \| Restricted Stock \| Below Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				50.00%
Fourth Anniversary \| Restricted Stock \| At or Above Manager Level
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage (as a percent)				33.30%