Preferred Stock and Common Stock - Narrative (Detail) (USD $)	0 Months Ended			3 Months Ended		6 Months Ended				3 Months Ended	6 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended			3 Months Ended	6 Months Ended
	Feb. 18, 2011	Jan. 07, 2011	Jul. 19, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010		Jun. 30, 2011 Common Stock Par Value	Jun. 30, 2011 Common Stock Par Value	Jun. 30, 2010 Common Stock Par Value	Jun. 23, 2011 Common Stock Par Value	Jun. 22, 2011 Common Stock Par Value	Jun. 30, 2011 Series A Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2010 Series A Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2011 Series A Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2010 Series A Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series A Cumulative Redeemable Preferred Stock [Member]		Jun. 23, 2011 Series A Cumulative Redeemable Preferred Stock [Member]	Jun. 30, 2011 Series B Cumulative Convertible Redeemable Preferred Stock [Member]	Jun. 30, 2010 Series B Cumulative Convertible Redeemable Preferred Stock [Member]	Jun. 30, 2011 Series B Cumulative Convertible Redeemable Preferred Stock [Member]	Jun. 30, 2010 Series B Cumulative Convertible Redeemable Preferred Stock [Member]	Dec. 31, 2010 Series B Cumulative Convertible Redeemable Preferred Stock [Member]		Jun. 23, 2011 Series B Cumulative Convertible Redeemable Preferred Stock [Member]	Jun. 30, 2011 Restricted Stock [Member]	Jun. 30, 2011 Restricted Stock [Member]
Class of Stock [Line Items]
Capital stock, par value				$ 0.01		$ 0.01		$ 0.01	[1]				$ 0.01
Capital stock, shares authorized													2,000,000,000	1,000,000,000
Common stock, shares authorized				1,987,987,500		1,987,987,500							1,987,987,500
Preferred Stock, shares authorized															7,412,500		7,412,500		7,412,500	[1]	7,412,500	4,600,000		4,600,000		4,600,000	[1]	4,600,000
Common stock issued - public offering	86,250,000	86,250,000	60,000,000
Net proceeds from common stock offering	$ 1,470,000,000	$ 1,470,000,000	$ 1,000,000,000			$ 2,941,411,000
Options exercised under incentive plans, shares				279,127	57,000	462,470	147,579
Aggregate exercise price of options exercised under incentive plans				3,700,000	753,000	6,100,000	1,800,000
Direct Purchase and Dividend Reinvestment Program - value raised				454,564,000	640,000	455,706,000	116,158,000
Direct Purchase and Dividend Reinvestment Program - issued shares				26,090,380	38,000	26,154,175	6,500,000
Shares granted under incentive plans, shares						7,500	7,500																						324,674	328,550
Number of common stock issued upon conversion of Preferred Stock, shares										2,732	8,045	1,511										1,000		3,000	645
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, ratio															2		2		2			2		2		2
Minimum ratio of votes required to materially and adversely change the terms of the Series A Preferred Stock															0.667		0.667		0.667			0.667		0.667		0.667
Preferred Stock shares issued															7,412,500		7,412,500		7,412,500	[1]		1,649,047		1,649,047		1,652,047	[1]
Preferred Stock shares outstanding															7,412,500		7,412,500		7,412,500	[1]		1,649,047		1,649,047		1,652,047	[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%		6.00%
Preferred Stock redeemable price, per share															$ 25.00		$ 25.00		$ 25.00			$ 25.00		$ 25.00		$ 25.00
Convertible Preferred Stock conversion trigger, common stock cash dividends per share excess																						$ 0.11		$ 0.11		$ 0.11
Minimum number of quarters with failure to pay dividends, which triggers voting rights for preferred stock, quarters															6		6		6
Initial conversion rate of convertible Preferred Stock, shares																						1.7730		1.7730
Conversion rate of convertible Preferred Stock, shares																						2.8152		2.8152
Common stock - dividend declared - total				540,000,000	380,600,000	1,038,665,000	744,411,000
Common stock - dividend declared - per share				$ 0.65	$ 0.68	$ 1.27	$ 1.33
Preferred stock dividends						$ 7,298,000	$ 7,297,000								$ 3,600,000	$ 3,600,000	$ 7,300,000	$ 7,300,000				$ 618,000	$ 976,000	$ 1,200,000	$ 2,000,000
Preferred series dividends declared, per share															$ 0.4922	$ 0.4922	$ 0.9844	$ 0.9844				$ 0.375	$ 0.375	$ 0.75	$ 0.75

[1] Derived from the audited consolidated financial statements at December 31, 2010.