Common Stock and Preferred Stock - Narrative (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015		Aug. 31, 2015
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	924,853,133		935,929,561	[1]
Common Stock, shares outstanding	924,853,133		935,929,561	[1]
Options exercised under incentive plans, shares	0	0
Direct purchase and dividend reinvestment program - value raised	$ 524,000	$ 570,000
Direct purchase and dividend reinvestment program - issued shares	54,000	53,000
Common stock repurchase program, authorized amount					$ 1,000,000,000
Common stock repurchase program, repurchased share	11,132,226
Common stock repurchase program, repurchased value	$ 102,700,000
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	2012-03
Number of common shares authorized for issuance under the Distribution Agency Agreement	125,000,000	125,000,000
Dividends payable, date to be paid	Apr. 29, 2016	Apr. 30, 2015
Common stock - dividend declared	$ 277,456,000	$ 284,310,000
Dividends Declared Per Share of Common Stock	$ 0.30	$ 0.30
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	$ 5,719,000	$ 5,719,000
Preferred series dividends declared, per share	$ 0.477	$ 0.477
7.625% Series C Cumulative Redeemable Preferred Stock | Preferred Stock Dividends
Class of Stock [Line Items]
Preferred series dividends declared, per share		$ 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	$ 8,625,000	$ 8,625,000
Preferred series dividends declared, per share	$ 0.469	$ 0.469
7.50% Series D Cumulative Redeemable Preferred Stock | Preferred Stock Dividends
Class of Stock [Line Items]
Preferred series dividends declared, per share		$ 0.469
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	$ 3,648,000	$ 3,648,000
Preferred series dividends declared, per share	$ 0.492	$ 0.492
7.875% Series A Cumulative Redeemable Preferred Stock | Preferred Stock Dividends
Class of Stock [Line Items]
Preferred series dividends declared, per share		$ 0.492
6.00% Series B Cumulative Convertible Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	4,600,000

[1] Derived from the audited consolidated financial statements at December 31, 2015.