Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended		1 Months Ended	9 Months Ended			0 Months Ended	9 Months Ended		1 Months Ended	9 Months Ended	12 Months Ended		0 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended	12 Months Ended		9 Months Ended		12 Months Ended	1 Months Ended
	Oct. 16, 2012	Oct. 29, 2012	Sep. 30, 2012	Sep. 30, 2013 Director	Sep. 30, 2012	Sep. 30, 2013 Director	Sep. 30, 2012	Dec. 31, 2012 Director		Oct. 31, 2013 Dividend Paid	Sep. 30, 2013 6.00% Series B Cumulative Convertible Preferred Stock	Sep. 30, 2012 6.00% Series B Cumulative Convertible Preferred Stock	Sep. 30, 2012 Common Stock	May 16, 2012 7.625% Series C Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.625% Series C Cumulative Redeemable Preferred Stock	Sep. 30, 2012 7.625% Series C Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.625% Series C Cumulative Redeemable Preferred Stock Dividend Paid	Sep. 30, 2013 Series C Preferred Stock Director	Dec. 31, 2012 Series C Preferred Stock Director	May 31, 2012 Series C Preferred Stock	Sep. 13, 2012 7.50% Series D Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.50% Series D Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.50% Series D Cumulative Redeemable Preferred Stock Dividend Paid	Sep. 30, 2013 Series D Preferred Stock Director	Dec. 31, 2012 Series D Preferred Stock Director	Sep. 30, 2012 Series D Preferred Stock	Sep. 30, 2013 7.875% Series A Cumulative Redeemable Preferred Stock	Sep. 30, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Sep. 30, 2013 7.875% Series A Cumulative Redeemable Preferred Stock Dividend Paid
Class of Stock [Line Items]
Options exercised under incentive plans, shares						166,375	443,669
Aggregate exercise price of options exercised under incentive plans						$ 2,200,000	$ 6,100,000
Direct purchase and dividend reinvestment program - issued shares						157,000	117,000
Direct purchase and dividend reinvestment program - value raised						2,166,000	1,979,000
Number of common stock issued upon conversion of Preferred Stock, shares												1,300,000	4,000,000
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
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		$ 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				12,650,000							18,400,000					7,412,500
Preferred Stock dividend rate, percentage											6.00%				7.625%			7.625%	7.625%			7.50%		7.50%	7.50%		7.875%		7.875%
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	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	2
Minimum ratio of votes required to materially and adversely change the terms of preferred stock				0.667		0.667		0.667										0.667	0.667					0.667	0.667
Common stock - dividend declared			487,200			1,136,626,000	1,557,537,000			331,600												1,136,626,000
Dividends Declared Per Share of Common Stock		$ 0.50		$ 0.35	$ 0.50	$ 1.20	$ 1.60			$ 0.35
Preferred dividends declared												$ 289,000			$ 17,156,000	$ 8,579,000	$ 5,700,000					$ 25,875,000	$ 8,600,000				$ 10,945,000	$ 10,945,000		$ 3,600,000
Preferred series dividends declared, per share												$ 0.375			$ 1.430	$ 0.715	$ 0.477					$ 1.406	$ 0.469				$ 1.477	$ 1.477		$ 0.492

[1] Derived from the audited consolidated financial statements at December 31, 2012.