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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
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- Definition

Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Subparagraph (j)
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- Definition

Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Paragraph 2
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

+ References

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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.

+ References

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-Publisher FASB
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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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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 2
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Paragraph 2
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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.

+ References

No definition available.

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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Mar. 31, 2015	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				$ 38	$ 52	$ 38
Cash received from the exercise of stock options				32	47	35
Tax benefit related to exercise of stock options				11	16	12
Total compensation expense related to stock incentive plans				28	27	25
Tax benefit related to compensation costs				19	8	8
Net of tax unrealized gains (losses) related to credit-only impaired securities	$ 52			$ 52	51	58
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)	9,300,000.0			9,300,000.0
Common Stock options issued under stock incentive plan (shares)				0
Annual Equity Bonus Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other benefit plans, shares issued (shares)		40,336	54,732
Common Stock issued, fair value per share (USD per share)		$ 71.05	$ 62.55
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	$ 16			$ 16
Weighted average period of cost expected to be recognized				2 years 3 months 18 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	24			$ 24
Weighted average period of cost expected to be recognized				2 years 3 months 18 days
Capital surplus
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit related to exercise of stock options					$ 12	$ 9
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	$ 9			$ 9