Fair Value Measurements (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Fair value measurements
Mortgage loans receivable	$ 171,321,000	$ 165,444,000
Senior unsecured notes	281,633,000	255,800,000
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Term Notes Due on or before 14 January 2019 [Member]
Fair value measurements
Discount rate (as a percent)	3.95%	3.95%
Fair Value, Inputs, Level 3 [Member] | Senior Unsecured Notes Maturing Beyond 2020 [Member]
Fair value measurements
Discount rate (as a percent)	4.80%	4.25%
Fair Value, Inputs, Level 3 [Member] | Mortgage Receivable [Member]
Fair value measurements
Discount rate (as a percent)	8.30%	8.40%
Reported Value Measurement [Member]
Fair value measurements
Mortgage loans receivable	171,321,000	165,444,000
Bonds payable	1,400,000	2,035,000
Bank borrowings	19,500,000	21,000,000
Senior unsecured notes	281,633,000	255,800,000
Estimate of Fair Value Measurement [Member]
Fair value measurements
Bonds payable	1,400,000	2,035,000
Bank borrowings	19,500,000	21,000,000
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair value measurements
Mortgage loans receivable	202,816,000	200,248,000
Senior unsecured notes	$ 280,899,000	$ 262,351,000