Incentive Plan (Summary of Restricted Stock Activity) (Details) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Units
Granted (shares)	120,368	128,986	73,449
Weighted-Average Grant Date Fair Value
Award vesting period	4 years
Restricted stock
Units
Unvested beginning of period (shares)	299,215
Granted (shares)	226,902
Vested (shares)	(111,950)
Cancelled or expired (shares)	(41,375)
Unvested end of period (shares)	372,792	299,215
Weighted-Average Grant Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 97.55
Granted (in dollars per share)	115.25	$ 100.33	$ 108.65
Vested (in dollars per share)	93.38
Cancelled or expired (in dollars per share)	107.52
Unvested, end of period (in dollars per share)	$ 108.47	$ 97.55

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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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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