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The net of tax unrealized gains or losses related to securities for which only the credit portion of an other-than-temporary impairment has been recorded in earnings.

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Restricted common stock vesting period.

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

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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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 tax benefit for recognition of expense of award under share-based payment arrangement.

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Amount of tax benefit from exercise of option under share-based payment arrangement.

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

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Description of service or performance condition required to be met for earning right to award under share-based payment arrangement. Includes, but is not limited to, combination of market, performance or service condition.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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Number of shares authorized for issuance under share-based payment arrangement.

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

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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, before forfeiture, of shares issued under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

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Amount of increase (decrease) in accumulated other comprehensive income (AOCI) for reclassification to retained earnings of tax effect from remeasurement of deferred tax pursuant to Tax Cuts and Jobs Act.

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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock vesting period				four-year
Rate of options exercisable per year, commencing one year after grant				20% per year commencing one year after grant
Duration of options expiration, after date of grant				10 years
Total intrinsic value of options exercised				$ 46	$ 57	$ 65
Cash received from the exercise of stock options				31	29	34
Tax benefit related to exercise of stock options				8	9	18
Total compensation expense related to stock incentive plans				23	23	30
Tax benefit related to compensation costs				13	13	27
Net of tax unrealized gains (losses) related to credit-only impaired securities	$ 68	$ 68		$ 55	$ 58	68
Cumulative effect of accounting change							$ 4
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect						$ 0
Accounting Standards Update 2018-02 \| Tax Cuts and Jobs Act of 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax Cuts and Jobs Act, reclassification from AOCI to retained earnings, tax effect	$ 145	$ 145
Equity securities \| Accounting Standards Update 2016-01
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative effect of accounting change							$ (221)
Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures				0	0	0
Common Stock reserved for issuance under stock incentive plans (shares)				4,500,000
Annual Equity Bonus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other benefit plans, shares issued (shares)			45,804
Common Stock issued, fair value per share (USD per share)			$ 115.49
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock vested during period, fair value				$ 11	$ 10	$ 14
Unrecognized compensation expense related to equity-based awards that have yet to vest				$ 36
Weighted average period of cost expected to be recognized				2 years 4 months 24 days
Maximum \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to equity-based awards that have yet to vest				$ 1