Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended			12 Months Ended			1 Months Ended	0 Months Ended	12 Months Ended					0 Months Ended	12 Months Ended					12 Months Ended			0 Months Ended	12 Months Ended
	Jan. 29, 2013	Oct. 16, 2012	Jan. 27, 2012	Dec. 31, 2013 Director Quarter	Dec. 31, 2012 Director Quarter	Dec. 31, 2011	Jan. 31, 2014 Dividend Paid	May 16, 2012 7.625% Series C Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.625% Series C Cumulative Redeemable Preferred Stock	Dec. 31, 2012 7.625% Series C Cumulative Redeemable Preferred Stock	Dec. 31, 2013 Series C Preferred Stock Quarter Director	Dec. 31, 2012 Series C Preferred Stock Director Quarter	May 31, 2012 Series C Preferred Stock	Sep. 13, 2012 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2012 7.50% Series D Cumulative Redeemable Preferred Stock	Dec. 31, 2013 Series D Preferred Stock Director Quarter	Dec. 31, 2012 Series D Preferred Stock Quarter Director	Sep. 30, 2012 Series D Preferred Stock	Dec. 31, 2013 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2011 7.875% Series A Cumulative Redeemable Preferred Stock	Jan. 03, 2012 6.00% Series B Cumulative Convertible Preferred Stock	Dec. 31, 2013 6.00% Series B Cumulative Convertible Preferred Stock	Dec. 31, 2012 6.00% Series B Cumulative Convertible Preferred Stock	Dec. 31, 2011 6.00% Series B Cumulative Convertible Preferred Stock	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock
Class of Stock [Line Items]
Options exercised under incentive plans, shares				166,375	603,169	679,000
Aggregate exercise price of options exercised under incentive plans				$ 2,200,000	$ 8,400,000	$ 9,000,000
Direct purchase and dividend reinvestment program - value raised				2,855,000	2,794,000	455,547,000
Direct purchase and dividend reinvestment program - issued shares				219,000	170,000	26,200,000
Number of common stock issued upon conversion of Preferred Stock, shares																									1,300,000	320,000.0	4,000,000	906,000.0
Description of common stock equity distribution agreement				On March 19, 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 19, 2012
Number of common shares authorized for issuance under the Distribution Agency Agreement				125,000,000	125,000,000
Shares of common stock reclassified to redeemable preferred stock								12,650,000						18,400,000
Preferred Stock, shares issued											12,000,000	12,000,000	12,000,000				18,400,000	18,400,000	18,400,000	7,412,500	7,412,500
Preferred Stock, par value											$ 0.01	$ 0.01	$ 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	$ 25.00	$ 25.00
Capital Stock, par value				$ 0.01	$ 0.01
Common stock, shares authorized				1,956,937,500	1,956,937,500
Preferred Stock, shares authorized									12,650,000						18,400,000					7,412,500				4,600,000
Preferred Stock dividend rate, percentage									7.625%		7.625%	7.625%			7.50%		7.50%	7.50%		7.875%	7.875%			6.00%
Common stock repurchase program, authorized amount		1,500,000,000
Common stock repurchase program, repurchased share					27,800,000
Common stock repurchase program, repurchased value					397,100,000
Common stock repurchase program, unsettled value					141,100,000
Preferred Stock, shares outstanding											12,000,000	12,000,000					18,400,000	18,400,000		7,412,500	7,412,500
Preferred Stock redeemable price, per share											$ 25.00	$ 25.00					$ 25.00	$ 25.00		$ 25.00	$ 25.00
Preferred Stock redemption date											May 16, 2017						Sep. 13, 2017			Apr. 05, 2009
Minimum number of quarters with failure to pay dividends, which triggers voting rights for preferred stock, quarters				6	6						6	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	2					2	2
Minimum ratio of votes required to materially and adversely change the terms of preferred stock				At least two-thirds							At least two-thirds						At least two-thirds
Common stock - dividend declared	432,200,000		552,800,000	1,420,856,000	1,989,690,000	2,173,222,000	284,200
Dividends Declared Per Share of Common Stock				$ 1.50	$ 2.05	$ 2.44	$ 0.30
Preferred dividends declared									$ 22,875,000	$ 14,297,000					$ 34,500,000	$ 10,351,000				$ 14,593,000	$ 14,593,000	$ 14,593,000	$ 2,300,000		$ 289,000	$ 2,261,000
Preferred series dividends declared, per share									$ 1.91	$ 1.19					$ 1.88	$ 0.56				$ 1.97	$ 1.97	$ 1.97	$ 1.50		$ 0.375	$ 1.50