Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 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 Units
Nonvested at the beginning of the period (in shares)		895,489
Granted (in shares)		313,984
Forfeitures (in shares)		(62,865)
Undistributed vested units (in shares)		(217,350)
Vested and distributed (in shares)		(176,043)
Nonvested at the end of the period (in shares)	753,215	753,215
Fully Vested Undistributed to Retirement-eligible employees (in shares)	548,542	548,542	712,572
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)		$ 52.28
Weighted-average Fair Value per Unit, Granted (in dollars per share)		62.88
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)		50.78
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)		53.57
Weighted-average Fair Value per Unit, Vested (in dollars per share)		52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 56.31	$ 56.31
Performance period		3 years
Award Vesting Period		38 months
Award Requisite Service Period		5 years
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Risk free interest rate period		3 years
Three-year risk-free rate		1.98%
Volatility rate, minimum		15.00%
Volatility rate, maximum		23.00%
Restricted Stock Units (RSUs) [Member]
Share Units
Nonvested at the beginning of the period (in shares)		0
Granted (in shares)		186,728
Forfeitures (in shares)		(4,964)
Undistributed vested units (in shares)		(19,742)
Vested and distributed (in shares)		0
Nonvested at the end of the period (in shares)	162,022	162,022
Fully Vested Undistributed to Retirement-eligible employees (in shares)	19,742	19,742
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)		$ 0.00
Weighted-average Fair Value per Unit, Granted (in dollars per share)		57.65
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)		58.99
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)		59.01
Weighted-average Fair Value per Unit, Vested (in dollars per share)		0.00
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 57.44	$ 57.44
Award Vesting Period		38 months
Award Requisite Service Period		5 years

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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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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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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