Fair Value Measurements (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Value Measurements
Mortgage loans receivable	$ 39,741,000	$ 39,299,000
Bonds payable	2,035,000	2,635,000
Bank borrowings	117,500,000	115,500,000
Senior unsecured notes	185,800,000	185,800,000
Contingent liabilities	6,854,000	6,744,000
Discount rate used to value future cash inflows of the mortgage loans receivable (as a percent)	6.00%	6.00%
Discount rate used to value earn out liabilities (as a percent)	6.50%	6.60%
Senior unsecured notes maturing prior to 2019
Value Measurements
Discount rate used to value future cash outflow (as a percent)	3.80%	3.80%
Senior unsecured notes maturing 2021
Value Measurements
Discount rate used to value future cash outflow (as a percent)	4.30%	4.30%
Carrying Value
Value Measurements
Mortgage loans receivable	39,741,000	39,299,000
Bonds payable	2,035,000	2,635,000
Bank borrowings	117,500,000	115,500,000
Senior unsecured notes	185,800,000	185,800,000
Contingent liabilities	6,854,000	6,744,000
Fair Value
Value Measurements
Bonds payable	2,035,000	2,635,000
Bank borrowings	117,500,000	115,500,000
Fair Value | Level 3
Value Measurements
Mortgage loans receivable	45,060,000	44,939,000
Senior unsecured notes	194,475,000	194,838,000
Contingent liabilities	$ 6,854,000	$ 6,744,000