Noncontrolling Interests of the Trust- Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Redeemable Noncontrolling Interest [Line Items]
Common units, units outstanding not held by the trust	3,539,075	3,553,566
Market value of common units based on closing price	$ 126.3

- Definition

The fair value as of the reporting date of noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. The noncontrolling interest holder's ownership (or holders' ownership) may be in the form of common shares (regardless of class), limited partnership units (regardless of class), non-preferential membership interests, or any other form of common equity regardless of investee entity legal form.

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