Federal and State Income Taxes - Summary Cash Dividends Per Series Preferred Share for Individual Shareholder (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2013 Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2012 Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2011 Series B-1 [Member]	Dec. 31, 2011 Series C [Member]	Dec. 31, 2013 Ordinary Dividends [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2012 Ordinary Dividends [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2011 Ordinary Dividends [Member] Series B-1 [Member]	Dec. 31, 2011 Ordinary Dividends [Member] Series C [Member]	Dec. 31, 2013 Capital Gains 15% Rate [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2012 Capital Gains 15% Rate [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2011 Capital Gains 15% Rate [Member] Series B-1 [Member]	Dec. 31, 2011 Capital Gains 15% Rate [Member] Series C [Member]	Dec. 31, 2013 Nontaxable Distribution [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2012 Nontaxable Distribution [Member] Series D Preferred Shares of Beneficial Interest [Member]	Dec. 31, 2011 Nontaxable Distribution [Member] Series B-1 [Member]	Dec. 31, 2011 Nontaxable Distribution [Member] Series C [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share	$ 0.40625	$ 2.31	$ 2.31	$ 1.30	$ 1.30	$ 2.31	$ 2.31	$ 1.30	$ 1.30