Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended		3 Months Ended		9 Months Ended				0 Months Ended	9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended				9 Months Ended		12 Months Ended
	Oct. 31, 2013	Oct. 16, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013		Oct. 31, 2014 Subsequent Event [Member]	Sep. 30, 2014 7.875% Series A Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.875% Series A Cumulative Redeemable Preferred Stock		Sep. 30, 2014 7.625% Series C Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.625% Series C Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.625% Series C Cumulative Redeemable Preferred Stock		Sep. 30, 2014 7.50% Series D Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock		Sep. 30, 2014 7.50% Series D Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Common Stock, par value			$ 0.01		$ 0.01		$ 0.01	[1]
Common Stock, shares authorized			1,956,937,500		1,956,937,500		1,956,937,500	[1]
Preferred Stock, shares authorized			4,600,000		4,600,000					7,412,500		7,412,500	[1]	12,650,000		12,650,000	[1]	18,400,000		18,400,000	[1]	18,400,000
Common Stock, shares issued			947,591,766		947,591,766		947,432,862	[1]
Common Stock, shares outstanding			947,591,766		947,591,766		947,432,862	[1]
Options exercised under incentive plans, shares					0	166,375
Aggregate exercise price of options exercised under incentive plans					$ 0	$ 2,200,000
Direct purchase and dividend reinvestment program - value raised					1,786,000	2,166,000
Direct purchase and dividend reinvestment program - issued shares					159,000	157,000
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	125,000,000	125,000,000
Preferred Stock, shares issued										7,412,500		7,412,500	[1]	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	[1]	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%		7.875%		7.625%		7.625%		7.50%				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				Sep. 13, 2017
Common stock - dividend declared	331,600,000				852,786,000	1,136,626,000			284,300,000
Dividends declared per share of common stock	$ 0.35		$ 0.30	$ 0.35	$ 0.90	$ 1.20			$ 0.30
Preferred dividends declared										$ 10,900,000	$ 10,900,000			$ 17,200,000	$ 17,200,000			$ 25,900,000	$ 25,900,000			$ 25,875,000	$ 25,875,000
Preferred Series dividends declared, per share										$ 1.477	$ 1.477			$ 1.430	$ 1.430			$ 1.406	$ 1.406			$ 1.406	$ 1.406

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