Income per Common Share of the Trust Share Repurchase (Details) - 2017 Share Repurchase Program [Member] - Common shares [Member] $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount \| $	$ 250	$ 250
Stock Repurchased During Period, Shares \| shares	0	0

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration