Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)	6 Months Ended			12 Months Ended
	Jun. 30, 2017		Jun. 30, 2016	Dec. 31, 2016		Jul. 12, 2016	Aug. 31, 2015
Class of Stock [Line Items]
Common Stock, par value	$ 0.01	[1]		$ 0.01	[2]
Common Stock, shares authorized	1,945,437,500	[1]		1,945,437,500	[2]
Common Stock, shares issued	1,019,027,880	[1]		1,018,913,249	[2]
Common Stock, shares outstanding	1,019,027,880	[1]		1,018,913,249	[2]
Options exercised under incentive plans, shares	0		0
Direct purchase and dividend reinvestment program - value raised	$ 1,270,000		$ 1,176,000
Direct purchase and dividend reinvestment program - issued shares of common stock	113,000		116,000
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
7.875% Series A Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	7,412,500	[1]		7,412,500	[2]
Preferred Stock, shares issued	7,412,500	[1]		7,412,500	[2]
Preferred Stock, shares outstanding	7,412,500	[1]		7,412,500	[2]
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
7.625% Series C Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	12,650,000	[1]		12,650,000	[2]
Preferred Stock, shares issued	12,000,000	[1]		12,000,000	[2]
Preferred Stock, shares outstanding	12,000,000	[1]		12,000,000	[2]
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
7.50% Series D Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	18,400,000	[1]		18,400,000	[2]
Preferred Stock, shares issued	18,400,000	[1]		18,400,000	[2]
Preferred Stock, shares outstanding	18,400,000	[1]		18,400,000	[2]
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
7.625% Series E Cumulative Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	11,500,000	[1]		11,500,000	[2]
Preferred Stock, shares issued	11,500,000	[1]		11,500,000	[2]
Preferred Stock, shares outstanding	11,500,000	[1]		11,500,000	[2]
Preferred Stock, par value	$ 0.01					$ 0.01
Preferred Stock liquidation preference, per share	$ 25.00
Preferred Stock dividend rate, percentage	7.625%
Preferred Stock redeemable price, per share	$ 25.00
Preferred Stock redemption date	Aug. 27, 2017
6.00% Series B Cumulative Convertible Preferred Stock
Class of Stock [Line Items]
Preferred Stock, shares authorized	4,600,000

[1] As a result of a change to a clearing organization's rulebook effective January 3, 2017, beginning with the first quarter 2017 and in subsequent periods the Company is presenting the fair value of centrally cleared interest rate swaps net of variation margin pledged under such transactions. The variation margin was previously reported under cash and cash equivalents and is currently reported as a reduction to interest rate swaps, at fair value. Balances reported prior to the effective date balances will not be adjusted.

[2] Derived from the audited consolidated financial statements at December 31, 2016.