Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Carrying Amount
Financial Assets:
Cash and cash equivalents	$ 73,390	$ 116,241
Mortgage loan receivable	3,374	3,417
Interest rate swap agreements	434	1,131
Financial Liabilities:
Mortgage notes payable	1,201,806	1,339,450
Notes payable to banks	600,000	481,500
Interest rate swap agreements	9,927	11,077
Fair Value
Financial Assets:
Cash and cash equivalents	73,390	116,241
Mortgage loan receivable	3,374	3,417
Interest rate swap agreements	434	1,131
Financial Liabilities:
Mortgage notes payable	1,206,540	1,327,637
Notes payable to banks	598,459	477,967
Interest rate swap agreements	$ 9,927	$ 11,077