Income per Common Share of the Trust (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income (Numerator) [Abstract]
Net income available to common shareholders	$ 51,411	$ 49,627	$ 94,443	$ 107,181
Weighted Average Shares (Denominator) [Abstract]
Weighted Average Number of Shares Outstanding, Basic	146,688	145,995	146,602	146,002
Dilutive shares for long-term compensation plans	820	740	753	620
Weighted Average Number of Shares Outstanding, Diluted	147,508	146,735	147,355	146,622
Common shares [Member]
Earnings Per Share, Basic [Abstract]
Income per common share - basic	$ 0.35	$ 0.34	$ 0.64	$ 0.73
Earnings Per Share, Diluted [Abstract]
Income per common share - diluted	$ 0.35	$ 0.34	$ 0.64	$ 0.73

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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