Common stock issued during period, fair value per share.

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- Definition

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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- Definition

Restricted common stock vesting period.

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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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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No definition available.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

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- Definition

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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- Definition

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

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-Name Accounting Standards Codification
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- Definition

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

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- Definition

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

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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- Definition

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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No definition available.

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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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- Definition

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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- Definition

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

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- Definition

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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- Definition

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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No definition available.

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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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			$ 65	$ 38	$ 52
Cash received from the exercise of stock options			33	32	47
Tax benefit related to exercise of stock options			18	11	16
Total compensation expense related to stock incentive plans			30	28	27
Tax benefit related to compensation costs			27	19	8
Net of tax unrealized gains (losses) related to credit-only impaired securities			68	$ 52	51
Impact of the U.S. corporate tax rate change on AOCI			$ 0			$ 2
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)			8,000,000.0
Common Stock options issued under stock incentive plan (shares)			0	0
Annual Equity Bonus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other benefit plans, shares issued (shares)	47,826	40,336
Common Stock issued, fair value per share (USD per share)	$ 96.13	$ 71.05
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			$ 7
Weighted average period of cost expected to be recognized			1 year 8 months 12 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to equity-based awards that have yet to vest			$ 31
Weighted average period of cost expected to be recognized			2 years 7 months 6 days
Capital surplus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit related to exercise of stock options					$ 12
Capital surplus under previous accounting guidance \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit related to exercise of stock options			$ 17	$ 9
Accounting Standards Update 2018-02 \| Tax Cuts and Jobs Act of 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Impact of the U.S. corporate tax rate change on AOCI			$ 145