Stock-based Compensation - Summary of Activity in the Trust's Restricted Share Units (Details) - $ / shares	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Weighted Average Grant Date Fair Value
Common stock, shares issued (in shares)	182,027,309	181,440,051
2013 Plan \| Executive Awards
Restricted Share Units
Non-vested at the beginning of the period (in shares)	354,123
Granted (in shares)	254,282
Vested (in shares)	(75,250)
Non-vested at the end of the period (in shares)	533,155
Weighted Average Grant Date Fair Value
Non-vested at beginning of period (in dollars per share)	$ 26.30
Granted (in dollars per share)	16.58
Vested (in dollars per share)	21.16
Non-vested at end of period (in dollars per share)	$ 22.66
Common stock, shares issued (in shares)	126,108
Restricted stock shares issued net of tax withholdings (in shares)	56,502
2013 Plan \| Trustee Awards
Restricted Share Units
Non-vested at the beginning of the period (in shares)	51,220
Granted (in shares)	50,745
Vested (in shares)	(34,807)
Non-vested at the end of the period (in shares)	67,158
Weighted Average Grant Date Fair Value
Non-vested at beginning of period (in dollars per share)	$ 19.04
Granted (in dollars per share)	14.78
Vested (in dollars per share)	18.67
Non-vested at end of period (in dollars per share)	$ 16.01

Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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The number of restricted shares issued as compensation, net of shares for the payment of withholding taxes. This element is to be used only if shares are used in lieu of cash to satisfy all or a portion of withholding taxes.

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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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-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
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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 number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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