Federal and State Income Taxes - Summary Cash Dividends Per Series Preferred Share for Individual Shareholder (Detail) (Series D Preferred Shares of Beneficial Interest [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share	$ 26.64	$ 2.31	$ 2.31
Ordinary Dividends [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share		$ 2.31	$ 2.31
Capital Gains [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share	$ 1.16
Nontaxable Distribution [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share
Cash Liquidation Distribution [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Dividends paid or accrued per Preferred Share	$ 25.48