Unit Option Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Unit options, outstanding	622,316	684,822
Unit options, exercised	(109,575)	(49,656)
Unit options, forfeited	(5,751)	(12,850)
Unit options, outstanding	506,990.00	622,316
Unit options exercisable	478,688	476,043
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Unit options outstanding, weighted average exercise price	$ 34.03	$ 33.97
Unit options exercised, weighted average exercise price	31.79	32.93
Unit options forfeited, weighted average exercise price	35.58	34.96
Unit options outstanding, weighted average exercise price	34.50	34.03
Unit options exercisable, weighted average exercise price	$ 34.36	$ 33.45

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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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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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.

+ References

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

+ References

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-Publisher FASB
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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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-Publisher FASB
-Name Accounting Standards Codification
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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 price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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

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