Income per Common Share of the Trust - Share Repurchase (Details) - 2017 Share Repurchase Program - Common shares - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized amount	$ 250	$ 250
Stock repurchased in period (in shares)	0	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.

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Name:	us-gaap_StockRepurchasedDuringPeriodShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration