Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended			12 Months Ended				12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended									12 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Mar. 01, 2012 Maximum [Member]	Feb. 28, 2011 Maximum [Member]	Dec. 31, 2012 ATM [Member]	Dec. 31, 2011 ATM [Member]	Dec. 31, 2010 ATM [Member]	May 04, 2010 ATM [Member]	Dec. 31, 2010 Drip [Member]	May 27, 2004 Series F Preferred Stock [Member]	Dec. 31, 2012 Series F Preferred Stock [Member]	Dec. 31, 2012 Series F Preferred Stock [Member]	May 27, 2004 Series G Preferred Stock [Member]	Dec. 31, 2012 Series G Preferred Stock [Member]	Dec. 31, 2012 Series G Preferred Stock [Member] Scenario, Forecast [Member]	Dec. 31, 2012 Common stock [Member]	Dec. 31, 2011 Common stock [Member]	Dec. 31, 2012 Common stock [Member] ATM [Member]	Dec. 31, 2011 Common stock [Member] ATM [Member]	Dec. 31, 2010 Common stock [Member] ATM [Member]	Mar. 01, 2012 Common stock [Member] ATM [Member]	Feb. 28, 2011 Common stock [Member] ATM [Member]	Dec. 31, 2010 Common stock [Member] Drip [Member]	Dec. 31, 2012 Series F Cumulative Preferred Stock [Member]	Jan. 13, 2006 Series J Preferred Stock [Member]	Dec. 31, 2012 Series J Preferred Stock [Member]	Dec. 31, 2012 Series J Preferred Stock [Member]	Dec. 31, 2011 Series J Preferred Stock [Member]	Dec. 31, 2010 Series J Preferred Stock [Member]	Aug. 21, 2006 Series K Preferred Stock [Member]	Dec. 31, 2012 Series K Preferred Stock [Member]	Dec. 31, 2011 Series K Preferred Stock [Member]	Dec. 31, 2010 Series K Preferred Stock [Member]	Dec. 31, 2012 Convertible Common Stock [Member]	Dec. 31, 2011 Convertible Common Stock [Member]	Dec. 31, 2010 Convertible Common Stock [Member]
Class of Stock [Line Items]
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%
Depositary share percentage per share										$ 0.01			$ 0.01												$ 0.0001					$ 0.0001
Par value of each depository shares										$ 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
Minimum 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											5.285%	5.285%
Redemption price per share											$ 1,000.00	$ 1,000.00			$ 1,000.00											$ 25.00	$ 25.00				$ 25.00
Aggregate redemption price	$ (50,044)	$ (50,044)									$ 50,000	$ 50,000			$ 25,000
Dividend equivalent to per depositary share														$ 72.36
Floating rates during floating rate periods														2.50%
Redemption date of preferred stock															Mar. 31, 2014
Preferred stock dividend exception rate																											8.25%
Depositary Shares redeemed																											2,000,000
Dividend per share, Depositary Shares																										$ 0.407812	$ 2.2657	$ 1.3594	$ 1.8125		$ 2.2657	$ 1.3594	$ 1.8125
Total Dividend, Depositary Shares																										816
Cost associated with share redemption	1,804	1,804																								1,804
Preferred stock shares authorized	10,000,000	10,000,000
Units exchanged for Stock																																		535,026	125,784	27,586
Noncontrolling interest reclassified to equity																																		4,763	1,109	316
Underwritten public offering																9,400,000	17,300,000
Total expenses related to underwriting																127	2,370
Net proceeds from underwriting																116,715	201,150
Shares of company stock issuable under an at-the-market offering			12,500,000	10,000,000				10,000,000
Issuance of company common stock					1,532,598	115,856	5,469,767		875,402
Proceeds from Common Stock Issuance ATM or DRIP					18,063	1,391	44,117		5,970
Sale agent commission																		369	28	900
Aggregate gross sale proceeds																					125,000	100,000
Shares awarded to directors																							23,567
Common stock approx fair value									$ 128