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Incentive Plans (Summary of Restricted Stock Units Activity) (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($) $ / shares shares
Time-Based Restricted Stock Units
Number of Shares (in shares)
Beginning balance (in shares) | shares	182,795
Granted (in shares) | shares	55,074
Vested and issued (in shares) | shares	(45,917)
Forfeited (in shares) | shares	(1,013)
Ending balance (in shares) | shares	190,939
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share) | $ / shares	$ 77.38
Granted (in dollars per share) | $ / shares	132.63
Vested and issued (in dollars per share) | $ / shares	71.95
Forfeited (in dollars per share) | $ / shares	80.06
Ending balance (in dollars per share) | $ / shares	$ 94.61
Total Fair Value
Vested and issued | $	$ 6.6
Outstanding | $	$ 27.6
Performance Units
Number of Shares (in shares)
Beginning balance (in shares) | shares	129,340
Granted (in shares) | shares	64,600
Vested and issued (in shares) | shares	(58,302)
Ending balance (in shares) | shares	135,638
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share) | $ / shares	$ 97.01
Granted (in dollars per share) | $ / shares	145.91
Vested and issued (in dollars per share) | $ / shares	76.78
Ending balance (in dollars per share) | $ / shares	$ 117.42
Total Fair Value
Vested and issued | $	$ 7.3
Outstanding | $	$ 19.6
Restricted Stock Units | 2013 Stock Plan For Non-Employee Directors
Number of Shares (in shares)
Beginning balance (in shares) | shares	31,004
Granted (in shares) | shares	8,692
Vested and issued (in shares) | shares	(9,589)
Ending balance (in shares) | shares	30,107
Weighted Average Grant Date Fair Value (in dollars per share)
Beginning balance (in dollars per share) | $ / shares	$ 83.31
Granted (in dollars per share) | $ / shares	125.04
Vested and issued (in dollars per share) | $ / shares	69.37
Ending balance (in dollars per share) | $ / shares	$ 99.79
Total Fair Value
Vested and issued | $	$ 1.2
Outstanding | $	$ 4.4

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Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, Outstanding, Fair Value

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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 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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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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