Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended	3 Months Ended				0 Months Ended	3 Months Ended				3 Months Ended							3 Months Ended
	Oct. 16, 2012	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013		Apr. 30, 2014 Subsequent Event [Member]	Mar. 31, 2014 7.875% Series A Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.875% Series A Cumulative Redeemable Preferred Stock		Mar. 31, 2014 7.625% Series C Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.625% Series C Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.625% Series C Cumulative Redeemable Preferred Stock		Mar. 31, 2014 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock		Mar. 31, 2014 Series D Preferred Stock	Mar. 31, 2013 Series D Preferred Stock	Mar. 31, 2014 7.50% Series D Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Common Stock, par value		$ 0.01		$ 0.01	[1]
Common Stock, shares authorized		1,956,937,500		1,956,937,500	[1]
Preferred Stock, shares authorized		4,600,000					7,412,500		7,412,500	[1]	12,650,000		12,650,000	[1]	18,400,000	18,400,000	[1]
Common Stock, shares issued		947,488,945		947,432,862	[1]
Common Stock, shares outstanding		947,488,945		947,432,862	[1]
Options exercised under incentive plans, shares		0	20,000
Aggregate exercise price of options exercised under incentive plans		$ 0	$ 300,000
Direct purchase and dividend reinvestment program - value raised		607,000	761,000
Direct purchase and dividend reinvestment program - issued shares		56,000	50,000
Common stock repurchase program, authorized amount	1,500,000,000
Common stock repurchase program, repurchased share			0
Common stock repurchase program, repurchased value			0
Description of common stock equity distribution agreement		In March 2012, the Company entered into six separate Distribution Agency Agreements ("Distribution Agency Agreements") with each of Merrill Lynch, Pierce, Fenner & Smith Incorporated, Credit Suisse Securities (USA) LLC, Goldman, Sachs & Co., J.P. Morgan Securities LLC, Morgan Stanley & Co. LLC and RCap Securities, Inc. (together, the Agents).
Date of Distribution Agency Agreement with six separate Agents		March 2012
Number of common shares authorized for issuance under the Distribution Agency Agreement		125,000,000	125,000,000
Preferred Stock, shares issued							7,412,500	7,412,500	7,412,500	[1]	12,000,000	12,000,000	12,000,000	[1]	18,400,000	18,400,000	[1]	18,400,000	18,400,000
Preferred Stock, shares outstanding							7,412,500	7,412,500	7,412,500	[1]	12,000,000	12,000,000	12,000,000	[1]	18,400,000	18,400,000	[1]	18,400,000	18,400,000
Preferred Stock, par value							$ 0.01	$ 0.01			$ 0.01	$ 0.01						$ 0.01	$ 0.01
Preferred Stock liquidation preference, per share							$ 25.00	$ 25.00			$ 25.00	$ 25.00						$ 25.00	$ 25.00
Preferred Stock dividend rate, percentage							7.875%				7.625%	7.625%						7.50%	7.50%
Preferred Stock redeemable price, per share							$ 25.00	$ 25.00			$ 25.00	$ 25.00						$ 25.00	$ 25.00
Preferred Stock redemption date							Apr. 05, 2009				May 16, 2017							Sep. 13, 2017
Common stock - dividend declared		284,247,000	426,173,000
Dividends Declared Per Share of Common Stock		$ 0.30	$ 0.45			$ 0.30
Preferred dividends declared							$ 3,648,000	$ 3,648,000			$ 5,719,000	$ 5,719,000						$ 8,625,000	$ 8,625,000	$ 8,600,000	$ 8,625,000
Preferred series dividends declared, per share							$ 0.492	$ 0.492			$ 0.477	$ 0.477						$ 0.469	$ 0.469	$ 0.469	$ 0.469

[1] Derived from the audited consolidated financial statements at December 31, 2013.