Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Tenant and other receivables, allowance for doubtful accounts	$ 1,061	$ 1,142
Accrued rental income, allowance	1,219	1,499
Unsecured senior notes, discount	$ 11,497	$ 12,296
Redeemable partnership units, series four preferred units outstanding (in units)	0	12,667
Redeemable partnership units, common units (in units)	16,192,518	16,453,670
Redeemable partnership units, long term incentive units outstanding at redemption value (in units)	1,839,555	1,496,799
General Partners' Capital Account, Units Outstanding (in units)	1,715,060	1,710,644
Limited Partners' Capital Account, Units Outstanding (in units)	151,758,871	151,403,301
Series B Cumulative Redeemable Preferred Stock [Member]
Series B Dividend Rate Percentage	5.25%	5.25%
Series B, Liquidation Preference Per Unit (dollars per share)	$ 2,500.00	$ 2,500.00
Series B, Units Issued	80,000	80,000
Series B, Units Outstanding (in shares)	80,000	80,000