Equity Based Compensation Plans Summary of stock options activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares [Roll Forward]
Beginning balance, outstanding	536,208	557,648	525,094
Granted	119,361	164,677	207,429
Exercised	(39,175)	(176,489)	(138,054)
Forfeited and canceled	(3,440)	(9,628)	(36,821)
Ending balance, outstanding	612,954	536,208	557,648
Options exercisable at year end	322,837	223,796	290,340
Weighted-average exercise price [Abstract]
Outstanding at beginning of year (in dollars per share)	$ 211.41	$ 181.50	$ 154.98
Granted (in dollars per share)	262.09	240.56	219.60
Exercised (in dollars per share)	159.05	146.86	138.79
Forfeited and canceled (in dollars per share)	221.80	160.40	178.18
Outstanding at end of year (in dollars per share)	224.57	211.41	181.50
Options exercisable at year end (in dollars per share)	$ 206.63	$ 191.09	$ 160.90

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

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Net number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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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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Number of share options (or share units) exercised during the current period.

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