Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended		0 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended				12 Months Ended		3 Months Ended	12 Months Ended		0 Months Ended	1 Months Ended
	Oct. 16, 2012	Apr. 26, 2012	Mar. 31, 2013 Director	Mar. 31, 2012	Dec. 31, 2012 Director		May 16, 2012 7.625% Series C Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.625% Series C Cumulative Redeemable Preferred Stock	Sep. 13, 2012 7.50% Series D Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.50% Series D Cumulative Redeemable Preferred Stock	Apr. 02, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Mar. 31, 2013 7.875% Series A Cumulative Redeemable Preferred Stock	Mar. 31, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2012 7.875% Series A Cumulative Redeemable Preferred Stock	Apr. 02, 2012 6.00% Series B Cumulative Convertible Preferred Stock	Mar. 31, 2013 6.00% Series B Cumulative Convertible Preferred Stock	Mar. 31, 2012 6.00% Series B Cumulative Convertible Preferred Stock	Mar. 31, 2012 Common Stock	Mar. 31, 2013 Series C Preferred Stock Director	Dec. 31, 2012 Series C Preferred Stock Director	May 31, 2012 Series C Preferred Stock	Mar. 31, 2013 Series D Preferred Stock Director	Dec. 31, 2012 Series D Preferred Stock Director	Sep. 30, 2012 Series D Preferred Stock	Apr. 29, 2013 Dividend Paid	Apr. 01, 2013 Dividend Paid 7.625% Series C Cumulative Redeemable Preferred Stock	Apr. 01, 2013 Dividend Paid 7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Options exercised under incentive plans, shares			20,000	126,394
Aggregate exercise price of options exercised under incentive plans			$ 265,000	$ 1,800,000
Direct purchase and dividend reinvestment program - value raised			761,000
Direct purchase and dividend reinvestment program - issued shares			50,000
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
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
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					12,000,000	12,000,000	12,000,000	18,400,000	18,400,000	18,400,000
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	[1]
Common Stock, shares authorized			1,956,937,500		1,956,937,500	[1]
Preferred Stock, shares authorized								12,650,000		18,400,000		7,412,500				4,600,000
Preferred Stock dividend rate, percentage								7.625%		7.50%		7.875%		7.875%		6.00%			7.625%	7.625%		7.50%	7.50%
Preferred Stock, shares outstanding												7,412,500		7,412,500					12,000,000	12,000,000		18,400,000	18,400,000
Preferred Stock redeemable price, per share												$ 25.00		$ 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	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			0.667		0.667														0.667	0.667		0.667	0.667
Common stock - dividend declared		534,401,000	426,173,000	534,401,000						426,173,000															426,173,000
Common stock - dividend declared - per share			$ 0.45	$ 0.55
Preferred dividends declared								$ 5,719,000		$ 8,625,000	$ 3,648,000	$ 3,648,000	$ 3,648,000		$ 289,000		$ 289,000									$ 5,719,000	$ 8,625,000
Preferred series dividends declared, per share								$ 0.477		$ 0.469		$ 0.492	$ 0.492				$ 0.375

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