Shareholders' Equity (Details)					3 Months Ended	6 Months Ended
	Jul. 22, 2016 USD ($) $ / shares	May 23, 2016 USD ($) $ / shares	May 17, 2016 USD ($) trustee shares	Feb. 25, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)
Distributions
Cash distribution to common shareholders (in dollars per share) | $ / shares		$ 0.43		$ 0.43
Distribution payable to common shareholders (in dollars per share) | $ / shares	$ 0.43
Distribution paid to common shareholders	$ 30,590,000	$ 30,585,000		$ 30,584,000
Cumulative Other Comprehensive Income (Loss)
Balance at the beginning of the period					$ 2,548,000	$ (14,867,000)
Other comprehensive loss before reclassifications					9,839,000	27,255,000
Amounts reclassified from cumulative other comprehensive loss to net income					4,000	3,000
Net current period other comprehensive loss					9,843,000	27,258,000
Balance at the end of the period					12,391,000	12,391,000
Unrealized Loss on Investment in Available for Sale Securities
Cumulative Other Comprehensive Income (Loss)
Balance at the beginning of the period					3,480,000	(9,391,000)
Other comprehensive loss before reclassifications					7,237,000	20,108,000
Net current period other comprehensive loss					7,237,000	20,108,000
Balance at the end of the period					10,717,000	10,717,000
Equity in Unrealized Gain (Loss) of an Investee
Cumulative Other Comprehensive Income (Loss)
Balance at the beginning of the period					(932,000)	(5,476,000)
Other comprehensive loss before reclassifications					2,602,000	7,147,000
Amounts reclassified from cumulative other comprehensive loss to net income					4,000	3,000
Net current period other comprehensive loss					2,606,000	7,150,000
Balance at the end of the period					$ 1,674,000	$ 1,674,000
Share Award Plan | Trustees
Share Awards
Number of Trustees | trustee			5
Number of shares granted to each trustee under the award plan | shares			2,500
Aggregate market value of shares granted under the Award Plan			$ 19.52
Number of Shares
Shares granted (in shares) | shares			2,500