Income Taxes (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Tax treatment of distributions:
Ordinary income (in dollars per share)	$ 2.23124	$ 2.28131	$ 1.99928
Long-term capital gain (in dollars per share)	$ 0.18884	$ 0.01869	$ 0.07644
Unrecaptured Section 1250 gain (in dollars per share)	$ 0.05992	$ 0.00000	$ 0.06428
Distribution reported for 1099-DIV purposes (in dollars per share)	$ 2.48000	$ 2.30000	$ 2.14000
REIT distribution requirement (as a percent)	90.00%	90.00%	90.00%
Provision (benefit) for income taxes
Current	$ 1,208,000	$ (4,080,000)	$ 2,459,000
Deferred	(7,490,000)	(26,580,000)	2,742,000
Income tax expense (benefit)	(6,282,000)	(30,660,000)	5,201,000
Income Tax Reconciliation, Noncontrolling Interest Income (Expense)			2,300,000
Canadian income tax benefit included in consolidated benefit for income taxes	$ (700,000)	$ (500,000)	$ (300,000)