Income Taxes - Federal Income Tax Treatment of Share Distributions (Detail) (USD $)	12 Months Ended		3 Months Ended		12 Months Ended				3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2013 Common Stock	Dec. 31, 2013 Series J Preferred Stock [Member]	Jun. 30, 2012 Series J Preferred Stock [Member]	Dec. 31, 2013 Series J Preferred Stock [Member]		Dec. 31, 2012 Series J Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member]	Sep. 30, 2013 Series K Preferred Stock [Member]	Dec. 31, 2013 Series K Preferred Stock [Member]		Dec. 31, 2012 Series K Preferred Stock [Member]	Dec. 31, 2011 Series K Preferred Stock [Member]	Dec. 31, 2012 Depositary Shares [Member] Series J Preferred Stock [Member]
Income Tax [Line Items]
Ordinary Income		$ 0.3088			$ 0.5085	[1]	$ 0.0000	$ 0.3130		$ 0.9969	[2]	$ 0.0000	$ 0.3130	$ 0.0000	[3]
Long-term Capital Gains		$ 0.0000			$ 0.0000	[1]	$ 0.8025	$ 0.0000		$ 0.0000	[2]	$ 0.8025	$ 0.0000	$ 0.7864	[3]
Unrecaptured Section 1250 Gain		$ 0.0000			$ 0.0000	[1]	$ 0.0000	$ 0.0000		$ 0.0000	[2]	$ 0.0000	$ 0.0000	$ 0.0000	[3]
Return of Capital		$ 0.0000			$ 0.0000	[1]	$ 1.4632	$ 1.0402		$ 0.0000	[2]	$ 1.4632	$ 1.0402	$ 1.4339	[3]
Qualified Dividends		$ 0.0000			$ 0.0000	[1]	$ 0.0000	$ 0.0062		$ 0.0000	[2]	$ 0.0000	$ 0.0062	$ 0.0000	[3]
Preferred Distribution per Depositary Share		$ 0.3088	$ 0.407812	$ 0.055382	$ 0.5085	[1]	$ 2.2657	$ 1.3594	$ 0.090625	$ 0.9969	[2]	$ 2.2657	$ 1.3594	$ 2.2203	[3]
Ordinary Income as a Percentage of Distribution		100.00%			100.00%	[1]	0.00%	23.02%		100.00%	[2]	0.00%	23.02%	0.00%	[3]
Long-term Capital Gains as a Percentage of Distribution	35.42%	0.00%			0.00%	[1]	35.42%	0.00%		0.00%	[2]	35.42%	0.00%	35.42%	[3]
Unrecaptured Section 1250 Gain as a Percentage of Distribution		0.00%			0.00%	[1]	0.00%	0.00%		0.00%	[2]	0.00%	0.00%	0.00%	[3]
Return of Capital as a Percentage of Distribution	64.58%	0.00%			0.00%	[1]	64.58%	76.52%		0.00%	[2]	64.58%	76.52%	64.58%	[3]
Qualified Dividends as a Percentage of Distribution		0.00%			0.00%	[1]	0.00%	0.46%		0.00%	[2]	0.00%	0.46%	0.00%	[3]
Preferred Distribution, Percentage		100.00%			100.00%	[1]	100.00%	100.00%		100.00%	[2]	100.00%	100.00%	100.00%	[3]

[1] The remaining 4,000,000 Depositary Shares of the Series J Preferred Stock were redeemed on April 11, 2013. The 2013 redemption had no impact on the 2012 or 2011 allocations included in the table above.

[2] Schedule relates to the 2,000,000 Depositary Shares of the Series K Preferred Stock that were redeemed on July 18, 2013. The 2013 redemption had no impact on the 2012 or 2011 allocations included in the table above.

[3] Schedule relates to the 2,000,000 Depositary Shares of the Series J Preferred Stock that were redeemed on December 21, 2012. The 2012 redemption had no impact on the 2011 allocation.