Common Stock and Preferred Stock - Narrative (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Common Stock, par value	$ 0.01		$ 0.01	[1]
Common Stock, shares authorized	1,956,937,500		1,956,937,500	[1]
Common Stock, shares issued	947,698,431		947,643,079	[1]
Common Stock, shares outstanding	947,698,431		947,643,079	[1]
Options exercised under incentive plans, shares	0	0
Direct purchase and dividend reinvestment program - value raised	$ 570	$ 607
Direct purchase and dividend reinvestment program - issued shares	53,000	56,000
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	284,310	284,247
Dividends payable, date to be paid	Apr. 30, 2015	Apr. 30, 2014
Dividends Declared Per Share of Common Stock	$ 0.30	$ 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	[1]
Preferred Stock, shares issued	7,412,500		7,412,500	[1]
Preferred Stock, shares outstanding	7,412,500		7,412,500	[1]
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	3,600	3,600
Preferred series dividends declared, per share	$ 0.492	$ 0.492
6.00% Series B Cumulative Convertible Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	4,600,000
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	12,650,000		12,650,000	[1]
Preferred Stock, shares issued	12,000,000		12,000,000	[1]
Preferred Stock, shares outstanding	12,000,000		12,000,000	[1]
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	5,700	5,700
Preferred series dividends declared, per share	$ 0.477	$ 0.477
7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	18,400,000		18,400,000	[1]
Preferred Stock, shares issued	18,400,000		18,400,000	[1]
Preferred Stock, shares outstanding	18,400,000		18,400,000	[1]
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	$ 8,600	$ 8,600
Preferred series dividends declared, per share	$ 0.469	$ 0.469

[1] Derived from the audited consolidated financial statements at December 31, 2014.