Stockholders' Equity - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended					12 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended							0 Months Ended	1 Months Ended	3 Months Ended		12 Months Ended				1 Months Ended	3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 01, 2012 Maximum	Feb. 28, 2011 Maximum	Dec. 31, 2013 ATM [Member]	Dec. 31, 2012 ATM [Member]	Dec. 31, 2011 ATM [Member]	May 27, 2004 Series F Preferred Stock	Dec. 31, 2013 Series F Preferred Stock	Dec. 31, 2013 Series F Preferred Stock	Dec. 31, 2013 Series F Preferred Stock Minimum	May 27, 2004 Series G Preferred Stock [Member]	Dec. 31, 2013 Series G Preferred Stock [Member]	Dec. 31, 2013 Series G Preferred Stock [Member] Scenario, Forecast [Member]	Dec. 31, 2013 Common Stock	Dec. 31, 2012 Common Stock	Dec. 31, 2011 Common Stock	Mar. 01, 2012 Common Stock ATM [Member]	Feb. 28, 2011 Common Stock ATM [Member]	Apr. 11, 2013 Series J Preferred Stock [Member]	Jan. 13, 2006 Series J Preferred Stock [Member]	Dec. 31, 2013 Series J Preferred Stock [Member]	Jun. 30, 2012 Series J Preferred Stock [Member]	Dec. 31, 2013 Series J Preferred Stock [Member]		Dec. 31, 2012 Series J Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member]	Aug. 21, 2006 Series K Preferred Stock [Member]	Sep. 30, 2013 Series K Preferred Stock [Member]	Dec. 31, 2013 Series K Preferred Stock [Member]		Dec. 31, 2012 Series K Preferred Stock [Member]	Dec. 31, 2011 Series K Preferred Stock [Member]	Jul. 18, 2013 Series K Preferred Stock [Member]	Dec. 31, 2013 Convertible Common Stock [Member]	Dec. 31, 2012 Convertible Common Stock [Member]	Dec. 31, 2011 Convertible Common Stock [Member]
Class of Stock [Line Items]
DepositarySharePercentagePerShare																	$ 0.01				$ 0.01									$ 0.0001							$ 0.0001
Issue date of depositary shares																	May 27, 2004				May 27, 2004									Jan. 13, 2006							Aug. 21, 2006
Depositary Shares issued																	50,000				25,000									6,000,000							2,000,000
Dividend Rate Per Share																			6.236%			7.236%											7.25%						7.25%
Preferred Stock, Par or Stated Value Per Share																	$ 0.01				$ 0.01									$ 0.01							$ 0.01
Initial offering price per depository share																	$ 1,000.00				$ 1,000.00									$ 25.00							$ 25.00
Number of years of U.S treasury rate one																		30 years	30 years			30 years
Fixed coupon rate of preferred stock																		2.375%	2.375%	2.375%
Number of years of U.s treasury rate two																		10 years	10 years			10 years
LIBOR rate																		3-month LIBOR	3-month LIBOR			3-month LIBOR
New coupon rate of preferred stock																		6.065%	6.065%
Redemption price per share																		$ 1,000.00	$ 1,000.00				$ 1,000.00						$ 25.00	$ 25.00	$ 25.00		$ 25.00						$ 25.00				$ 25.00
Aggregate redemption price	$ (150,051,000)				$ (50,044,000)				$ (150,051,000)	$ (50,044,000)								$ 50,000,000	$ 50,000,000				$ 25,000,000							$ 150,000,000									$ 50,000,000
Dividend equivalent to per depositary share																						$ 72.36
Floating rates during floating rate periods																						2.50%
Redemption date of preferred stock																							Mar. 31, 2014
Depositary Shares redeemed																													4,000,000				2,000,000
Dividend per share, Depositary Shares																								$ 0.3088							$ 0.407812	$ 0.055382	$ 0.5085	[1]	$ 2.2657	$ 1.3594		$ 0.090625	$ 0.9969	[2]	$ 2.2657	$ 1.3594
Total Dividend, Depositary Shares																													221,000		816,000							181,000
Cost associated with share redemption	0	2,121,000	3,546,000	0	1,804,000	0	0	0	5,667,000	1,804,000	0																						3,546,000		1,804,000				2,121,000
Preferred stock shares authorized	10,000,000								10,000,000
Units exchanged for Stock																																												105,028	535,026	125,784
Noncontrolling interest reclassified to equity									0	0	0																																	996,000	4,763,000	1,109,000
Underwritten public offering																								8,400,000	9,400,000	17,300,000
Net proceeds from underwriting																								132,050,000	116,715,000	201,150,000
Shares of company stock issuable under an at-the-market offering												12,500,000	10,000,000
Issuance of company common stock														2,315,704	1,532,598	115,856
Proceeds from Common Stock Issuance ATM or DRIP														41,735,000	18,063,000	1,391,000
Aggregate gross sale proceeds																											$ 125,000,000	$ 100,000,000

[1] The remaining 4,000,000 Depositary Shares of the Series J Preferred Stock were redeemed on April 11, 2013. The 2013 redemption had no impact on the 2012 or 2011 allocations included in the table above.

[2] Schedule relates to the 2,000,000 Depositary Shares of the Series K Preferred Stock that were redeemed on July 18, 2013. The 2013 redemption had no impact on the 2012 or 2011 allocations included in the table above.