Stock Compensation Plan (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Options
Stock options awarded
Options granted (in shares)	108,420
Per share weighted average exercise price (in dollars per share)	$ 93.22
Per share weighted average grant date fair value (in dollars per share)	$ 15.00
Fair value assumptions
Expected term in years	5 years 8 months 12 days
Risk free interest rate (as a percent)	2.50%
Volatility (as a percent)	21.50%
Dividend yield (as a percent)	3.00%
Stock options and Stock Appreciation Rights vested
Options vested (in shares)	100,652
Aggregate grant date fair value of Options vested	$ 1.4
Outstanding Stock Options and Stock Appreciation Rights
Options outstanding (in shares)	537,246	464,958
Aggregate intrinsic value (in dollars)	$ 10.0	$ 16.3
Per share weighted average exercise price (in dollars per share)	$ 63.85	$ 55.60
Exercisable Options (in shares)	307,043	241,944
Aggregate intrinsic value (in dollars)	$ 9.2	$ 12.1
Unvested Options (in shares)	230,203	223,014
Per share weighted average grant date fair value (in dollars per share)	$ 14.23	$ 13.87
Performance Share Units ("PSUs")
Additional disclosures
Granted (in shares)	40,747
Percentage of target to be awarded, low end of range (not less than)	40.00%
Percentage of target to be awarded, high end of range (not more than)	200.00%
Market price at grant date of performance units (in dollars per share)	$ 93.21
Restricted Stock Units (RSUs)
Additional disclosures
Granted (in shares)	2,030
Equity instruments other than options, weighted average grant date fair value (USD per share)	$ 80.15
Equity instruments other than options, vesting period	1 year

The market price at the grant date for the Performance Units granted.

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Represents the amount of difference between the fair value of the underlying shares reserved for issuance and exercise price of vested portions of options and SARs outstanding and currently exercisable.

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Represents the number of shares into which the fully or partially vested stock options and SARs outstanding, as of the balance sheet date, can be currently converted under the plan.

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As of the balance sheet date, aggregate intrinsic value of fully vested and expected to vest options and stock appreciation rights outstanding.

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Represents the number of shares into which fully vested and expected to vest stock options and SARs outstanding can be converted under the option plan as of the balance sheet date.

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Represents the number of stock options and SARs that vested during the reporting period.

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Represents the aggregate grant date fair value of stock options and SARs that vested during the reporting period.

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Represents the high end of the range of the payout depending on the achievement of goals.

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Represents the low end of the range of the payout depending on the achievement of goals.

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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 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 estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the 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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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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