Distributions (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Distributions
Minimum distribution of taxable income (as a percent)									90.00%
Ordinary taxable distribution (in dollars per share)									$ 1.370	$ 1.200	$ 1.549
Return of capital (in dollars per share)									$ 0.295	$ 0.334
Unrecaptured Section 1250 gain (in dollars per share)										$ 0.034
Long term capital gain (in dollars per share)									$ 0.015	$ 0.012	$ 0.011
Total (in dollars per share)	$ 0.42	$ 0.42	$ 0.42	$ 0.42	$ 0.41	$ 0.39	$ 0.39	$ 0.39	$ 1.68	$ 1.58	$ 1.56