Equity (Details) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Class of Stock [Line Items]
Proceeds of Common Stock sold					$ 23,270,000	$ 2,998,000	$ 150,941,000
Dividends declared and paid per Common Stock (in dollars per share)					$ 1.70	$ 1.70	$ 1.70
Excess of Preferred Stock redemption/repurchase cost over carrying value	0	0	1,895,000	0	1,895,000	0	0
Dividends, Common Stock [Abstract]
Ordinary income					$ 1.15	$ 0.41	$ 1.09
Capital gains					$ 0.00	$ 0.44	$ 0.60
Return of capital					$ 0.55	$ 0.85	$ 0.01
Total taxability of dividends per share					$ 1.70	$ 1.70	$ 1.70
Warrants [Abstract]
Warrants to acquire Common Stock, outstanding	15,000				15,000	15,000
Exercise price of outstanding warrants	32.50				32.50	32.50
ATM Equity Offering [Member]
Class of Stock [Line Items]
Number of Common Stock sold at the market (in shares)					378,200
Average price of Common Stock sold at the market					$ 35.09
Net proceeds of Common Stock sold at the market					13,100,000
Issuance of Equity [Member]
Class of Stock [Line Items]
Number of Common Stock sold (in shares)							7,000,000
Proceeds of Common Stock sold							144,100,000
Series A Cumulative Redeemable Preferred Shares [Member]
Class of Stock [Line Items]
Issuance date, Preferred Stock					Feb. 12, 1997	Feb. 12, 1997
Number of shares outstanding, Preferred Stock (in shares)	29,077				29,077	29,092
Carrying value, Preferred Stock	29,077,000				29,077,000	29,092,000
Liquidation preference per share, Preferred Stock (in dollars per share)	$ 1,000				$ 1,000	$ 1,000
Optional redemption date, Preferred Stock					Feb. 12, 2027	Feb. 12, 2027
Annual dividends payable per share, Preferred Stock (in dollars per share)					$ 86.25	$ 86.25	$ 86.25
Dividends, Preferred Stock [Abstract]
Ordinary income					$ 86.25	$ 41.80	$ 55.86
Capital gains					$ 0.00	$ 44.45	$ 30.39
Total taxability of dividends per share					$ 86.25	$ 86.25	$ 86.25
Series B Cumulative Redeemable Preferred Shares [Member]
Class of Stock [Line Items]
Number of Preferred Stock redeemed during period (in shares)					2,100,000
Redemption price of Preferred Stock redeemed during period					52,500,000
Excess of Preferred Stock redemption/repurchase cost over carrying value					1,900,000
Issuance date, Preferred Stock						Sep. 25, 1997
Number of shares outstanding, Preferred Stock (in shares)	0				0	2,100,000
Carrying value, Preferred Stock	$ 0				$ 0	$ 52,500,000
Liquidation preference per share, Preferred Stock (in dollars per share)	$ 0				$ 0	$ 25
Optional redemption date, Preferred Stock						Sep. 25, 2002
Annual dividends payable per share, Preferred Stock (in dollars per share)					$ 1.05	$ 2.00	$ 2.00
Dividends, Preferred Stock [Abstract]
Ordinary income					$ 1.05	$ 0.97	$ 1.30
Capital gains					$ 0.00	$ 1.03	$ 0.70
Total taxability of dividends per share					$ 1.05	$ 2.00	$ 2.00