Income Taxes - Federal Income Tax Treatment of Share Distributions (Detail) (USD $)	12 Months Ended			3 Months Ended		12 Months Ended		3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2012		Sep. 30, 2013
Common Stock
Income Tax [Line Items]
Ordinary Income	$ 0.4412	$ 0.3088
Long-term Capital Gains	$ 0.0000	$ 0.0000
Unrecaptured Section 1250 Gain	$ 0.0000	$ 0.0000
Return of Capital	$ 0.0000	$ 0.0000
Qualified Dividends	$ 0.0000	$ 0.0000
Distributions Per Common Share	$ 0.4412	$ 0.3088
Ordinary Income as a Percentage of Distribution	100.00%	100.00%
Long-term Capital Gains as a Percentage of Distribution	0.00%	0.00%
Unrecaptured Section 1250 Gain as a Percentage of Distribution	0.00%	0.00%
Return of Capital as a Percentage of Distribution	0.00%	0.00%
Qualified Dividends as a Percentage of Distribution	0.00%	0.00%
Common Distributions Percentage	100.00%	100.00%
Series J Preferred Stock
Income Tax [Line Items]
Ordinary Income		$ 0.5085	[1]			$ 0.0000
Long-term Capital Gains		$ 0.0000	[1]			$ 0.8025
Unrecaptured Section 1250 Gain		$ 0.0000	[1]			$ 0.0000
Return of Capital		$ 0.0000	[1]			$ 1.4632
Qualified Dividends		$ 0.0000	[1]			$ 0.0000
Distributions Per Preferred Share		$ 0.5085	[1]	$ 0.055382	$ 0.407812	$ 2.2657
Ordinary Income as a Percentage of Distribution		100.00%	[1]			0.00%
Long-term Capital Gains as a Percentage of Distribution		0.00%	[1]			35.42%
Unrecaptured Section 1250 Gain as a Percentage of Distribution		0.00%	[1]			0.00%
Return of Capital as a Percentage of Distribution		0.00%	[1]			64.58%
Qualified Dividends as a Percentage of Distribution		0.00%	[1]			0.00%
Preferred Distributions Percentage		100.00%	[1]			100.00%
Series K Preferred Stock
Income Tax [Line Items]
Ordinary Income		$ 0.9969	[2]			$ 0.0000
Long-term Capital Gains		$ 0.0000	[2]			$ 0.8025
Unrecaptured Section 1250 Gain		$ 0.0000	[2]			$ 0.0000
Return of Capital		$ 0.0000	[2]			$ 1.4632
Qualified Dividends		$ 0.0000	[2]			$ 0.0000
Distributions Per Preferred Share		$ 0.9969	[2]			$ 2.2657		$ 0.090625
Ordinary Income as a Percentage of Distribution		100.00%	[2]			0.00%
Long-term Capital Gains as a Percentage of Distribution		0.00%	[2]			35.42%
Unrecaptured Section 1250 Gain as a Percentage of Distribution		0.00%	[2]			0.00%
Return of Capital as a Percentage of Distribution		0.00%	[2]			64.58%
Qualified Dividends as a Percentage of Distribution		0.00%	[2]			0.00%
Preferred Distributions Percentage		100.00%	[2]			100.00%
Depositary Shares | Series J Preferred Stock
Income Tax [Line Items]
Ordinary Income						$ 0.0000	[3]
Long-term Capital Gains						$ 0.7864	[3]
Unrecaptured Section 1250 Gain						$ 0.0000	[3]
Return of Capital						$ 1.4339	[3]
Qualified Dividends						$ 0.0000	[3]
Distributions Per Preferred Share						$ 2.2203	[3]
Ordinary Income as a Percentage of Distribution						0.00%	[3]
Long-term Capital Gains as a Percentage of Distribution						35.42%	[3]
Unrecaptured Section 1250 Gain as a Percentage of Distribution						0.00%	[3]
Return of Capital as a Percentage of Distribution						64.58%	[3]
Qualified Dividends as a Percentage of Distribution						0.00%	[3]
Preferred Distributions Percentage						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 allocation 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 allocation 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.