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Shareholders' Equity / Partners' Capital - 2012 Equity Incentive Plan Narrative (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Stock options and Restricted stock units
Class of Stock [Line Items]
Award vesting period	4 years	4 years
Restricted stock units | Non-Management Trustees
Class of Stock [Line Items]
Award vesting period	1 year
2012 Equity Incentive Plan | Stock options
Class of Stock [Line Items]
Granted (in shares)	20,000	140,000
Expiration period for stock options	10 years
2012 Equity Incentive Plan | Restricted stock units
Class of Stock [Line Items]
Restricted stock unit grants in period (in shares)	345,334	304,400

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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