Income Taxes - Summary of Tax Characterization of Common Share Dividends Per Share as Reported to Shareholders (Detail) (USD $)	3 Months Ended													12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	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, 2009	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Preference Dividend Paid Per Share Characterized For Tax [Line Items]
Date Paid	Jan. 04, 2013	Oct. 02, 2012	Jul. 06, 2012	Apr. 03, 2012	Jan. 06, 2012	Oct. 11, 2011	Jul. 06, 2011	Apr. 05, 2011	Jan. 05, 2011	Oct. 05, 2010	Jul. 07, 2010	Apr. 06, 2010	Jan. 06, 2010
Gross Ordinary Income	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Capital Gain Distributions	$ 0.0000	$ 0.0183	$ 0.0183	$ 0.0183	$ 0.0122	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0000	$ 0.0671	$ 0.0000	$ 0.0000
Return of Capital	$ 0.0000	$ 0.1017	$ 0.1017	$ 0.1017	$ 0.0678	$ 0.0600	$ 0.0400	$ 0.0400	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.3729	$ 0.1600	$ 0.0800
Total Dividends	$ 0.0000	$ 0.1200	$ 0.1200	$ 0.1200	$ 0.0800	$ 0.0600	$ 0.0400	$ 0.0400	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.0200	$ 0.4400	$ 0.1600	$ 0.0800