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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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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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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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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Retirement and Share-based Compensation Plans (Stock Option Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding and vested, aggregate intrinsic value	$ 28.8
Proceeds from stock options exercised	$ 14.0	$ 5.0	$ 8.1
Stock Option Plans, 1996 Stock Option Plan, 2000 Employee Stock Option Plan, 2000 Directors Stock Option Plan, 2011 Directors Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	3,475,000
Duration of award	10 years
Stock Option Plans, 1996 Stock Option Plan, 2000 Employee Stock Option Plan, 2000 Directors Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	6 months
Stock Option Plans, 2011 Director Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Number of shares available for future grant	18,000
Stock Option Plans, Independent Insurance Agent Stock Option Plans [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	8,000,000
Stock Option Plans, All Stock Option Plans For Officers, Directors And Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options exercised in period, total intrinsic value	$ 1.5	$ 4.0	$ 1.4