Common Stock and Preferred Stock - Narrative (Detail) (USD $)	0 Months Ended		12 Months Ended			1 Months Ended
	Jan. 29, 2015	Jan. 29, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2014	Oct. 16, 2012
Class of Stock [Line Items]
Common Stock, par value			$ 0.01	$ 0.01
Common Stock, shares authorized			1,956,937,500	1,956,937,500
Common Stock, shares issued			947,643,079	947,432,862
Common Stock, shares outstanding			947,643,079	947,432,862
Options exercised under incentive plans, shares			0	166,375	603,000
Aggregate exercise price of options exercised under incentive plans			$ 0	$ 2,200,000	$ 8,400,000
Direct purchase and dividend reinvestment program - value raised			2,370,000	2,855,000	2,794,000
Direct purchase and dividend reinvestment program - issued shares			210,000	219,000	170,000
Common stock repurchase program, authorized amount							1,500,000,000
Common stock repurchase program, repurchased share				0
Common stock repurchase program, repurchased value				0
Description of common stock equity distribution agreement			In March 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 2012
Number of common shares authorized for issuance under the Distribution Agency Agreement			125,000,000	125,000,000
Common stock - dividend declared	284,300,000	432,200,000	1,137,079,000	1,420,856,000	1,989,690,000
Dividends declared per share of common stock			$ 1.2	$ 1.5	$ 2.05
Common Stock
Class of Stock [Line Items]
Direct purchase and dividend reinvestment program - value raised			2,000	2,000	2,000
Common stock - dividend declared					4,000,000
Subsequent Event
Class of Stock [Line Items]
Common stock - dividend declared	284,300,000
Dividends declared per share of common stock	$ 0.30
Dividend Paid
Class of Stock [Line Items]
Common stock - dividend declared						284,200,000
Dividends declared per share of common stock						$ 0.30
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized			7,412,500	7,412,500
Preferred Stock, shares issued			7,412,500	7,412,500
Preferred Stock, shares outstanding			7,412,500	7,412,500
Preferred Stock, par value			$ 0.01	$ 0.01
Preferred Stock liquidation preference, per share			$ 25.00	$ 25.00
Preferred Stock dividend rate, percentage			7.875%	7.875%
Preferred Stock redeemable price, per share			$ 25.00	$ 25.00
Preferred Stock redemption date			Apr. 05, 2009
Preferred dividends declared			14,600,000	14,600,000
Preferred series dividends declared, per share			$ 1.97	$ 1.97
6.00% Series B Cumulative Convertible Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized			4,600,000
Common stock - dividend declared					1,300,000
Preferred dividends declared					289,000
Preferred series dividends declared, per share					$ 0.375
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized			12,650,000	12,650,000
Preferred Stock, shares issued			12,000,000	12,000,000
Preferred Stock, shares outstanding			12,000,000	12,000,000
Preferred Stock, par value			$ 0.01	$ 0.01
Preferred Stock liquidation preference, per share			$ 25.00	$ 25.00
Preferred Stock dividend rate, percentage			7.625%	7.625%
Preferred Stock redeemable price, per share			$ 25.00	$ 25.00
Preferred Stock redemption date			May 16, 2017
Preferred dividends declared			22,900,000	22,900,000
Preferred series dividends declared, per share			$ 1.91	$ 1.91
7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized			18,400,000	18,400,000
Preferred Stock, shares issued			18,400,000	18,400,000
Preferred Stock, shares outstanding			18,400,000	18,400,000
Preferred dividends declared			34,500,000	34,500,000
Preferred series dividends declared, per share			$ 1.88	$ 1.88
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares issued			18,400,000	18,400,000
Preferred Stock, shares outstanding			18,400,000	18,400,000
Preferred Stock, par value			$ 0.01	$ 0.01
Preferred Stock liquidation preference, per share			$ 25.00	$ 25.00
Preferred Stock dividend rate, percentage			7.50%	7.50%
Preferred Stock redeemable price, per share			$ 25.00	$ 25.00
Preferred Stock redemption date			Sep. 13, 2017
Preferred dividends declared			34,500,000	34,500,000
Preferred series dividends declared, per share			$ 1.88	$ 1.88
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared			14,593,000	14,593,000	14,593,000
Preferred series dividends declared, per share			$ 1.97	$ 1.97	$ 1.97
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared			22,875,000	22,875,000	14,297,000
Preferred series dividends declared, per share			$ 1.91	$ 1.91	$ 1.19
7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred dividends declared					$ 10,351,000
Preferred series dividends declared, per share					$ 0.56