Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended			3 Months Ended		9 Months Ended						1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended			0 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended	0 Months Ended	1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 15, 2011	Feb. 18, 2011	Jan. 07, 2011	Sep. 30, 2012 Director	Sep. 30, 2011	Sep. 30, 2012 Director	Sep. 30, 2011	Dec. 31, 2011 Director		Jun. 23, 2011	Jun. 22, 2011	Oct. 29, 2012 Dividend Paid [Member]	Oct. 27, 2011 Dividend Paid [Member]	Sep. 30, 2012 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2011 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2012 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2011 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Jun. 23, 2011 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Mar. 27, 2012 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Sep. 30, 2012 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Sep. 30, 2011 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Sep. 30, 2012 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Sep. 30, 2011 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Dec. 31, 2011 6.00% Series B Cumulative Convertible Preferred Stock [Member]		Jun. 23, 2011 6.00% Series B Cumulative Convertible Preferred Stock [Member]	May 16, 2012 7.625% Series C Cumulative Redeemable Preferred Stock [Member]	May 31, 2012 7.625% Series C Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2012 7.625% Series C Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2012 7.625% Series C Cumulative Redeemable Preferred Stock [Member]	Sep. 13, 2012 7.50% Series D Cumulative Redeemable Preferred Stock [Member]	Sep. 30, 2012 7.50% Series D Cumulative Redeemable Preferred Stock [Member]	Mar. 27, 2012 Common Stock Par Value	Sep. 30, 2011 Common Stock Par Value	Sep. 30, 2012 Common Stock Par Value	Sep. 30, 2011 Common Stock Par Value
Class of Stock [Line Items]
Options exercised under incentive plans, shares				50,000	134,000	443,669	596,470
Aggregate exercise price of options exercised under incentive plans				$ 669,000	$ 1,800,000	$ 6,100,000	$ 7,900,000
Number of common stock issued upon conversion of Preferred Stock, shares																				772,000		260,000.0	1,300,000	263,000.0										2,400,000	732,000.0	4,000,000	740,000.0
Direct Purchase and Dividend Reinvestment Program - value raised				1,100,000		1,979,000	455,706,000
Direct Purchase and Dividend Reinvestment Program - issued shares				65,000		117,000	26,200,000
Description of common stock equity distribution agreement						On March 19, 2012, the Company entered into six separate Distribution Agency Agreements (or 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 19, 2012
Number of common shares authorized for issuance under the Distribution Agency Agreement				125,000,000		125,000,000
Common stock issued - public offering	138,000,000	86,250,000	86,250,000
Net proceeds from common stock offering	2,400,000,000	1,500,000,000	1,500,000,000				5,351,846,000
Capital Stock, par value				$ 0.01		$ 0.01		$ 0.01	[1]	$ 0.01
Common stock, shares authorized										2,000,000,000	1,000,000,000
Common Stock, shares authorized				1,956,937,500		1,956,937,500		1,987,987,500	[1]	1,987,987,500
Preferred Stock, shares authorized														7,412,500		7,412,500			7,412,500		4,600,000		4,600,000		4,600,000	[1]	4,600,000			12,650,000	12,650,000		18,400,000
Shares of common stock reclassified to redeemable preferred stock																												12,650,000				18,400,000
Preferred Stock, shares issued														7,412,500		7,412,500		7,412,500			0		0		1,331,849	[1]			12,000,000				18,400,000
Preferred Stock, shares outstanding														7,412,500		7,412,500		7,412,500			0		0		1,331,849	[1]
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%							6.00%				7.625%				7.50%
Preferred Stock redeemable price, per share														$ 25.00		$ 25.00		$ 25.00											$ 25.00				$ 25.00
Preferred Stock redemption date																Apr. 05, 2009													May 16, 2017				Sep. 13, 2017
Minimum number of quarters with failure to pay dividends, which triggers voting rights for preferred stock, quarters				6		6		6
Number of Board Of Directors that Preferred Stock owners are entitled to vote to elect when there is a failure to pay quarterly dividends for a set period				2		2		2
Minimum ratio of votes required to materially and adversely change the terms of preferred stock				0.667		0.667		0.667
Convertible Preferred Stock conversion trigger, common stock cash dividends per share excess																									$ 0.11
Initial conversion rate of convertible Preferred Stock, shares																									1.7730
Conversion rate of convertible Preferred Stock, shares																									3.0614
Common stock - dividend declared				487,237,000	581,800,000	1,557,537,000	1,620,416,000					487,237,000	581,800,000
Common stock - dividend declared - per share				$ 0.50	$ 0.60	$ 1.60	$ 1.87
Preferred stock - dividends declared														$ 3,600,000	$ 3,600,000	$ 10,945,000	$ 10,945,000				$ 0	$ 524,000	$ 289,000	$ 1,761,000						$ 5,700,000	$ 8,579,000
Preferred series dividends declared, per share														$ 0.492	$ 0.492	$ 1.477	$ 1.477				$ 0.000	$ 0.375	$ 0.375	$ 1.125						$ 0.477	$ 0.715

[1] Derived from the audited consolidated financial statements at December 31, 2011.