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