Subsequent Events (Details) (USD $)			0 Months Ended					3 Months Ended					12 Months Ended	1 Months Ended	3 Months Ended					12 Months Ended		1 Months Ended	0 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Jan. 22, 2013 Common Shares	Jan. 15, 2013 Common Shares	Oct. 15, 2012 Common Shares	Jul. 13, 2012 Common Shares	Apr. 13, 2012 Common Shares	Mar. 31, 2013 Common Shares	Dec. 31, 2012 Common Shares	Sep. 30, 2012 Common Shares	Jun. 30, 2012 Common Shares	Mar. 31, 2012 Common Shares	Dec. 31, 2012 Common Shares	Nov. 30, 2011 Series A Preferred Stock	Mar. 31, 2013 Series A Preferred Stock	Dec. 31, 2012 Series A Preferred Stock	Sep. 30, 2012 Series A Preferred Stock	Jun. 30, 2012 Series A Preferred Stock	Mar. 31, 2012 Series A Preferred Stock	Dec. 31, 2012 Series A Preferred Stock	Nov. 02, 2011 Series A Preferred Stock	May 07, 2013 Subsequent event	May 06, 2013 Subsequent event Common Shares	Apr. 15, 2013 Subsequent event Common Shares	May 06, 2013 Subsequent event Series A Preferred Stock	Apr. 02, 2013 Subsequent event Series A Preferred Stock	May 06, 2013 Subsequent event Series B Preferred Stock	Apr. 16, 2013 Subsequent event Series B Preferred Stock
Subsequent Events
Dividend paid on preferred stock (in dollars per share)																										$ 0.5625
Dividends on common stock declared (in dollars per share)									$ 0.27	$ 0.27	$ 0.27	$ 0.26	$ 1.07										$ 0.30	$ 0.30
Number of shares sold			6,284,152											2,760,000														2,800,000
Dividend rate (as a percent)														9.00%														6.625%
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01																			$ 0.01							$ 0.01
Issue price (in dollars per share)			$ 18.30											$ 25.00														$ 25.00
Gross proceeds from sale of shares																												$ 70,000,000
New or additional borrowings																						0
Shares of common stock issued for services provided by directors				2,851	2,876	3,108	3,776	2,418					12,611											2,418
Fair value of shares of common stock issued for services provided by directors				54,000	49,000	46,000	50,000	52,000					199,000											52,000
Dividend declared but not paid (in dollars per share)															$ 0.5625	$ 0.5625	$ 0.5625	$ 0.5625	$ 0.5625	$ 2.25					$ 0.5625		$ 0.34505
Outstanding balance	$ 20,000,000	$ 99,300,000																				$ 0