Common Shares of Beneficial Interest (Narrative) (Details) - USD ($) $ in Millions	May 03, 2019	Mar. 31, 2019
Class of Stock [Line Items]
Value of shares approved to be repurchased		$ 200.0
Stock repurchase program, remaining authorized repurchase amount		$ 181.5
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorized repurchase amount	$ 181.5