Common stock issued during period, fair value per share.

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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 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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Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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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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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

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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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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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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Gross number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP). Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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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	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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				$ 57	$ 65	$ 38
Cash received from the exercise of stock options				29	34	32
Tax benefit related to exercise of stock options				9	18	11
Total compensation expense related to stock incentive plans				23	30	28
Tax benefit related to compensation costs				13	27	19
Net of tax unrealized gains (losses) related to credit-only impaired securities	$ 68			$ 58	68	$ 52
Impact of the U.S. corporate tax rate change on AOCI	4				$ 4
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock vesting period				3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock vesting period				4 years
Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock reserved for issuance under stock incentive plans (shares)				7,000,000.0
Common Stock options issued under stock incentive plan (shares)				0	0	0
Annual Equity Bonus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other benefit plans, shares issued (shares)		45,804	47,826
Common Stock issued, fair value per share (USD per share)		$ 115.49	$ 96.13
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				$ 3
Weighted average period of cost expected to be recognized				1 year
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Common Stock vested during period, fair value				$ 10	$ 14	$ 10
Unrecognized compensation expense related to equity-based awards that have yet to vest				$ 35
Weighted average period of cost expected to be recognized				2 years 6 months
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 of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect	$ 145
Equity securities \| Accounting Standards Update 2016-01
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Impact of the U.S. corporate tax rate change on AOCI							$ (221)