Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended				2 Months Ended	3 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012	Mar. 06, 2014	Dec. 31, 2012	May 27, 2004	Apr. 11, 2013	Dec. 21, 2012	Jan. 13, 2006	Aug. 21, 2006	Mar. 13, 2014	Mar. 01, 2012
Class of Stock [Line Items]
Cost associated with the redemptions	$ 0	$ 0	$ 0	$ 1,462	$ 0	$ 2,121	$ 3,546	$ 0	$ 1,462	$ 5,667		$ 1,804
Preferred stock shares authorized	10,000,000								10,000,000
Noncontrolling interest reclassified to equity									0	0		0
Net proceeds from the issuance of common stock									0	174,081		134,905
Series F Preferred Stock
Class of Stock [Line Items]
Depositary Shares issued															50,000
Depositary Share percentage per share															1.00%
Dividend rate per share									6.236%				6.275%
Par value per share															$ 0.01
Initial offering price per depository share															$ 1,000.00
Redemption price per share															$ 1,000.00
Aggregate redemption price															50,000
Dividend per Depositary share				$ 11.3299
Total dividends, Depositary Shares				566
Cost associated with the redemptions									949
Minimum fixed coupon rate of preferred stock	2.375%								2.375%
Number of years of U.S. treasury rate one													30 years
Number of years of U.S. treasury rate two													10 years
LIBOR rate													3-month LIBOR
Series G Preferred Stock
Class of Stock [Line Items]
Depositary Shares issued															25,000
Depositary Share percentage per share															1.00%
Dividend rate per share									7.236%
Par value per share															$ 0.01
Initial offering price per depository share															$ 1,000.00
Redemption price per share															$ 1,000.00
Aggregate redemption price															25,000
Dividend per Depositary share				$ 36.18
Total dividends, Depositary Shares				905
Cost associated with the redemptions									513
Series J Preferred Stock
Class of Stock [Line Items]
Depositary Shares issued																		6,000,000
Depositary Share percentage per share																		0.01%
Dividend rate per share									7.25%
Par value per share																		$ 0.01
Initial offering price per depository share																		$ 25.00
Redemption price per share																		$ 25.00
Aggregate redemption price																		150,000
Dividend per Depositary share							$ 0.055382			$ 0.5085	[1]	$ 2.2657		$ 0.407812
Total dividends, Depositary Shares							221							816
Cost associated with the redemptions										3,546		1,804
Depositary Shares redeemed																4,000,000	2,000,000
Series K Preferred Stock
Class of Stock [Line Items]
Depositary Shares issued																			2,000,000
Depositary Share percentage per share																			0.01%
Dividend rate per share									7.25%
Par value per share																			$ 0.01
Initial offering price per depository share																			$ 25.00
Redemption price per share																			$ 25.00
Aggregate redemption price																			50,000
Dividend per Depositary share						$ 0.090625				$ 0.9969	[2]	$ 2.2657
Total dividends, Depositary Shares						181
Cost associated with the redemptions										2,121
Common Stock
Class of Stock [Line Items]
Shares of common stock issued										8,400,000		9,400,000
Net proceeds from the issuance of common stock										132,050		116,715
Convertible Common Stock
Class of Stock [Line Items]
Units converted to common stock									222,676	105,028		535,026
Noncontrolling interest reclassified to equity									2,155	996		4,763
ATM
Class of Stock [Line Items]
Shares of common stock issued										2,315,704		1,532,598
Net proceeds from the issuance of common stock										41,735		18,063
Shares of company stock issuable under an at-the-market offering																				13,300,000	12,500,000
Aggregate gross sale proceeds																				$ 200,000	$ 125,000

[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 allocation 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 allocation included in the table above.