Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended				12 Months Ended					0 Months Ended	12 Months Ended							0 Months Ended	12 Months Ended				0 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	0 Months Ended
	Oct. 16, 2012	Jul. 15, 2011	Feb. 18, 2011	Jan. 07, 2011	Dec. 31, 2012 Director	Dec. 31, 2011 Director	Dec. 31, 2010	Jun. 23, 2011	Jun. 22, 2011	Mar. 27, 2012 Common Stock Par Value	Dec. 31, 2012 Common Stock Par Value	Dec. 31, 2011 Common Stock Par Value	Dec. 31, 2010 Common Stock Par Value	Dec. 31, 2012 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2011 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Dec. 31, 2010 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]	Dec. 31, 2012 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Dec. 31, 2011 6.00% Series B Cumulative Convertible Preferred Stock [Member]	Dec. 31, 2010 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]	Dec. 31, 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]	Dec. 31, 2012 7.50% Series D Cumulative Redeemable Preferred Stock [Member]	Jan. 27, 2012 Dividend Paid [Member]	Jan. 29, 2013 Dividend Paid [Member]	Jan. 03, 2012 Dividend Paid [Member] 7.875% Series A Cumulative Redeemable Preferred Stock [Member]	Jan. 03, 2012 Dividend Paid [Member] 6.00% Series B Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Options exercised under incentive plans, shares					603,169	678,920
Aggregate exercise price of options exercised under incentive plans					$ 8,400,000	$ 9,000,000
Number of common stock issued upon conversion of Preferred Stock, shares										2,400,000	4,000,000	906,000.0	2,400,000					772,000	1,300,000	320,000.0	953,000.0
Direct Purchase and Dividend Reinvestment Program - value raised					2,794,000	455,547,000	278,783,000
Direct Purchase and Dividend Reinvestment Program - issued shares					170,000	26,200,000
Description of common stock equity distribution agreement					On March 19, 2012, we 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
Common stock repurchase program, authorized amount	1,500,000,000
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)	(1,047,355,000)
Capital Stock, par value					$ 0.01	$ 0.01		$ 0.01
Capital stock, shares authorized								2,000,000,000	1,000,000,000
Common stock, shares authorized					1,956,937,500	1,987,987,500		1,987,987,500
Preferred Stock, shares authorized														7,412,500			7,412,500		4,600,000	4,600,000		4,600,000			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				0	1,331,849				12,000,000			18,400,000
Preferred Stock, shares outstanding														7,412,500	7,412,500				0	1,331,849
Preferred Stock, par value														$ 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
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
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
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
Minimum ratio of votes required to materially and adversely change the terms of preferred stock					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					1,989,690,000	2,173,222,000	1,570,674,000																					1,989,690,000	552,806,000	432,154,000
Common stock - dividend declared - per share					$ 2.05	$ 2.44	$ 2.65
Dividend distributions characterized for federal income tax purposes as ordinary income, percentage					89.30%	95.10%
Dividend distributions characterized for federal income tax purposes as long-term capital, percentage					10.70%	4.90%
Preferred dividends declared														$ 14,593,000	$ 14,593,000	$ 14,593,000			$ 289,000	$ 2,261,000	$ 3,440,000				$ 14,297,000			$ 10,351,000			$ 14,593,000	$ 2,261,000
Preferred series dividends declared, per share														$ 1.97	$ 1.97	$ 1.97			$ 0.375	$ 1.50	$ 1.50				$ 1.19			$ 0.56