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Share-Based Compensation - 2012 Equity Incentive Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2012
Stock options
Class of Stock [Line Items]
Vesting period		4 years	4 years
Total unrecognized compensation cost	$ 0.2
Weighted-average period of unvested cost	9 months 18 days
Restricted stock units
Class of Stock [Line Items]
Vesting period	3 years	4 years	4 years
Total unrecognized compensation cost	$ 10.3
Weighted-average period of unvested cost	1 year 9 months 18 days
Restricted stock units | Non-Management Trustees
Class of Stock [Line Items]
Vesting period	1 year	1 year
2012 Equity Incentive Plan | Stock options
Class of Stock [Line Items]
Expiration period	10 years
2012 Equity Incentive Plan | Class A Common Shares/units
Class of Stock [Line Items]
Shares available for issuance (in shares)				6,000,000

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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 difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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