Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Weighted-average Fair Value per Unit at Grant Date
Closing common share price (in dollars per share)		$ 58.99
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fully Vested Undistributed to Retirement-eligible employees.	403,584	712,572
Share Units
Share Units, Nonvested as of beginning of year	895,489
Share Units, Granted	298,774
Share Units, Forfeitures	(37,337)
Share Units, Undistributed Vested	(72,392)
Share Units, Vested	(176,043)
Share Units, Nonvested as of Current period end	908,491
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year	$ 52.28
Weighted-average Fair Value per Unit, Granted	62.88
Weighted-average Fair Value per Unit, Forfeitures	46.08
Weighted-average Fair Value per Unit, Undistributed Vested Units	53.50
Weighted-average Fair Value per Unit, Vested	52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end	$ 55.81
Performance period	3 years
Risk free interest rate period	3 years
Three-year risk-free rate	1.98%
Volatility rate, minimum	15.00%
Volatility rate, maximum	23.00%
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fully Vested Undistributed to Retirement-eligible employees.	6,458
Share Units
Share Units, Nonvested as of beginning of year	0
Share Units, Granted	181,145
Share Units, Forfeitures	(643)
Share Units, Undistributed Vested	(6,458)
Share Units, Vested	0
Share Units, Nonvested as of Current period end	174,044
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year	$ 0.00
Weighted-average Fair Value per Unit, Granted	57.62
Weighted-average Fair Value per Unit, Forfeitures	58.99
Weighted-average Fair Value per Unit, Undistributed Vested Units	58.99
Weighted-average Fair Value per Unit, Vested	0.00
Weighted-average Fair Value per Unit, Nonvested as of Current period end	$ 57.56

Share-based Compensation Arrangement by Share-based Payment Award, Performance Period

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fully Vested Undistributed to Retirement-eligible employees.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period but were undistributed.

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Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Period

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan that vested during the reporting period but were undistributed.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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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Price of a single share of a number of saleable stocks of a company.

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